Why did Zamasu use Goku's body?

[The Patrolman]

I mean he hates Goku so much and he is the reason why the Zero Mortal Plan exists. So why would he steal his body? Seems pretty hypocritical.

[Doctor.]

He explained why in episode #66.

[Merged Zamasu]

Black is aware of the fact it's incoherent to use a mortal body to carry out the Zero-mortals Plan. Yet, Black also desired the Sayan's ability to become stronger after each fight. When he was still Zamasu, Black didn't have that possibility, but after he became one with Goku's body, his potential expanded to no end, as he stated in Episode 64. Zamasu had an immortal soul, but he was not physically the strongest. Black realized that Goku's immense strenght was necessary to succeed in the Zero-mortals Plan. It wouldn't be the first time Black takes "incoherent" decisions to succeed in his Zero-mortals Plan. He also killed all Gods, despite his many statements that all he did was for the Gods. But they would thwart his plan, they were a threat and had to be dealt with. Another reason, as Doctor said, is the fact that Goku represents exactly the flaws of the Gods. The Gods, whom in their ignorance and incompetence allowed a mortal, like Goku, to obtain powers that surpassed even theirs as Gods. This was seen by Zamasu as the ultimate sin, that he would carry with him for the rest of his existence, as a reminder of the mistakes the Gods committed, and also as a reminder of what the duties of a God truly are: to keep mortals in-check, and make sure evil is defeat. Mortals, who do not learn from their mistakes (like the stubborn Goku), are evil.

[Saturnine]

It's not Goku who was the reason for the Zero Mortal Plan. It was Trunks.

[GodKaio-Ken]

It's not Goku who was the reason for the Zero Mortal Plan. It was Trunks.

What? How did you come up with that?

[Yomi]

He was pretty envious of Goku I'd say. Don't wanna be racist, but he IS green... lol.

Shin was pretty shocked too when he saw the Saiyans but he didn't go all crazy.

[Merged Zamasu]

It's not Goku who was the reason for the Zero Mortal Plan. It was Trunks.

What? How did you come up with that?

Black himself stated that in Episode 61. First off, Time-travel is a sin and forbidden by the Gods. This means that Trunks had committed a serious crime against the laws of the Gods, a crime that had to be punished, because Black, unlike Beerus, Supreme Kai and Gowasu, who are clearly incompetent fools, cares about the well-being of the Universe and takes seriously his duty as a God. Then, the very reason Black wanted Goku's body is because Trunks altered the past. Had Trunks not changed the Present timeline, Goku would have died because of the heart virus and Black would have never met him, as such he would have never wished for Goku's body.

[Freeza9000]

Because of the fact that Goku, despite being a mortal, managed to reach the levels of the gods and even siding with a God of Destruction, a privilege a mortal rarely gets. A mortal rivaling a God of Destruction's powers is quite impressive and in Zamasu's eyes, Goku, despite being a mere mortal, has basically done the impossible. Sure it is true that Goku hasn't gotten as strong as gods such as Whis, but Zamasu thought that he has the high potential to surpass all the gods. Hence, in EP 66, when Vegetto mocked Zamasu of stealing a mortal's body, Zamasu replied that Goku has "power that surpasses even gods". Basically, what Zamasu thought is that if he were to make Goku's body his own, he would have the power and ability to do the impossible.

[Gog]

Because, Toriyama needed an evil goku, without an actual evil Goku. Goku black was the amazing love child of the plot. But I dunno , there is maybe, probably, hopefully a good justification for his existence.
Maybe, Prehaps he wanted to take in the sins of the mortals?

[Doctor.]

It's not Goku who was the reason for the Zero Mortal Plan. It was Trunks.

What? How did you come up with that?

Black himself stated that in Episode 61. First off, Time-travel is a sin and forbidden by the Gods. This means that Trunks had committed a serious crime against the laws of the Gods, a crime that had to be punished, because Black, unlike Beerus, Supreme Kai and Gowasu, who are clearly incompetent fools, cares about the well-being of the Universe and takes seriously his duty as a God. Then, the very reason Black wanted Goku's body is because Trunks altered the past. Had Trunks not changed the Present timeline, Goku would have died because of the heart virus and Black would have never met him, as such he would have never wished for Goku's body.

Black pulled that out of his ass, he didn't even know that Trunks had a time machine. He discovered that fact AFTER he already had initiated his zero mortals plan.

[Merged Zamasu]

What? How did you come up with that?

Black himself stated that in Episode 61. First off, Time-travel is a sin and forbidden by the Gods. This means that Trunks had committed a serious crime against the laws of the Gods, a crime that had to be punished, because Black, unlike Beerus, Supreme Kai and Gowasu, who are clearly incompetent fools, cares about the well-being of the Universe and takes seriously his duty as a God. Then, the very reason Black wanted Goku's body is because Trunks altered the past. Had Trunks not changed the Present timeline, Goku would have died because of the heart virus and Black would have never met him, as such he would have never wished for Goku's body.

Black pulled that out of his ass, he didn't even know that Trunks had a time machine. He discovered that fact AFTER he already had initiated his zero mortals plan.

Perhaps, but Trunks is still indirectly responsible for Black's creation, because if he had let Goku die of heart virus, Zamasu would have never wished for his body. Black considers Humanity the epitome of evil in the Universe, he hates all mortals, but Humans in particular because, as you said, he found out the terrible sin Trunks had committed.

[Doctor.]

Perhaps, but Trunks is still indirectly responsible for Black's creation, because if he had let Goku die of heart virus, Zamasu would have never wished for his body. Black considers Humanity the epitome of evil in the Universe, he hates all mortals, but Humans in particular because, as you said, he found out the terrible sin Trunks had committed.

Trunks is indirectly responsible for Black's creation, but Trunks was absolutely not one of the reasons for the Zero Mortals Plan. It was Goku and Zamasu's preexisting bias against mortals. Black only discovered that Trunks could time travel mid-way through the arc, which fomented his hatred, and probably only had that entire speech to piss off Trunks before he killed him.

[SsjCookie]

Trunks may have been indirectly responsible for Blacks creation, but what would have been the alternative?
We all know that without Trunks intervention Goku would have died and so would everyone else with him.
The Androids would have ruled the world, Trunks and Bulma would have died as well eventually.

BUT to be honest, we just can't know for sure.
What would have happened if everything just played out the way it should have been?
Maybe humanity would have died out.... or not.
17 and 18 would eventually be absorbed by Cell and he would continue with earth's destruction......or not.
We just don't know.

The bittersweet thing is, Trunks rescued a different timeline, but lost his own by doing so.

[HeroR]

Black pulled that out of his ass, he didn't even know that Trunks had a time machine. He discovered that fact AFTER he already had initiated his zero mortals plan.

If he didn't know about Trunks' time travel, how did he know it was Trunks that saved Goku from the heart virus? Trunks never told him that detail.

[Doctor.]

Black pulled that out of his ass, he didn't even know that Trunks had a time machine. He discovered that fact AFTER he already had initiated his zero mortals plan.

If he didn't know about Trunks' time travel, how did he know it was Trunks that saved Goku from the heart virus? Trunks never told him that detail.

He probably asked Zamasu to investigate about it and they found out somehow. Black had no idea Trunks could go back in time in the beginning because he didn't even know Trunks had a time machine and was surprised when the time ring pulled him back to the past.

[Jinzoningen MULE]

Isn't it pretty likely that Zuno told Zamasu those details?

[Psykomatik]

He knows all of that thanks to Zuno.

[HeroR]

He probably asked Zamasu to investigate about it and they found out somehow. Black had no idea Trunks could go back in time in the beginning because he didn't even know Trunks had a time machine and was surprised when the time ring pulled him back to the past.

How would Future Zamasu know that? The only people who would know are Future Trunks and Bulma. Plus, Present Zamasu asked about everything about Goku from Zuno, so why wouldn't he know about Goku being saved from the heart virus by a time traveler named Trunks. He was surprised since he didn't know what the Time Machine looked like, the moment he realized he went into the past, he put two and two together quickly. He also wouldn't know that the Time Ring would take him to the past, even with Time Machine, since Time Rings normally can't travel back. Even Whis was surprised by this.

[Doctor.]

How would Future Zamasu know that? The only people who would know are Future Trunks and Bulma. Plus, Present Zamasu asked about everything about Goku from Zuno, so why wouldn't he know about Goku being saved from the heart virus by a time traveler named Trunks. He was surprised since he didn't know what the Time Machine looked like, the moment he realized he went into the past, he put two and two together quickly. He also wouldn't know that the Time Ring would take him to the past, even with Time Machine, since Time Rings normally can't travel back. Even Whis was surprised by this.

Like Jinzoningen MULE pointed out, they could have asked Zuno. Also, they're Gods, they may have ways to check the past. Whis seemed to know all about the Saiyans (that Goku defeated Freeza, that they lived on Earth and could all go Super Saiyan) and I doubt he was watching them since the beginning. He seemed surprised, to me, at the fact that Trunks could even time travel, not the fact that he had a machine.

[TheMikado]

Because, Toriyama needed an evil goku, without an actual evil Goku. Goku black was the amazing love child of the plot. But I dunno , there is maybe, probably, hopefully a good justification for his existence.
Maybe, Prehaps he wanted to take in the sins of the mortals?

That's the train I'm on, honestly it makes Zamasu seem incredibly petty to specifically kill all mortals AND Gods in ALL the universes because Goku was strong and embarrassed him.
I mean I personally don't remember any enemy being that petty and causing so much destruction over something so trivial. Even Cell seemed content to leave most of the Earth alone while he tested his strength at the Cell games.