How was it possible that Zamasu killed Future Goku?

[super michael]

When Zamasu made the wish to swap body with Goku, how was Zamasu (Goku body) able to kill Goku (Zamasu body) without transforming?
In the present time line Goku needed to transform into Super Saiyan 2 to defeat Zamasu, so how so come Zamasu (Goku body) base form was enough to kill Goku (Zamasu body).

Both of these two wouldn't be used to each other body, just think of what happened when Goku and Ginyu switched body, they needed time to get used to their body little by little.

[SupremeKai25]

The Meta reason: The writers wanted to show just how twisted Zamasu had become. Having Zamasu mercilessly slaughter Goku in cold blood before Goku can even understand what is going on is an effective way to do so. It shows just how cold-blooded Zamasu has become, how he murders people ruthlessly, and how screwed up/confused Goku was.

The In-universe reason: Goku was completely confused. Like imagine that you're a Saiyan. One day you're working in the field, talking to your son, and then suddenly you turn into a green elf. Wouldn't you be extremely confused about what just happened? Yes you would. Now you have to remember that in Dragon Ball and in real life when one of the two fighters is extremely confused/disoriented, they'll be defeated quickly, and that's what happened to Goku. Zamasu obviously wasn't confused because it was his wish to switch bodies with Goku.

And another thing, since Zamasu took a lot of time (1+ year) to unlock the full potential of Goku's body, the inverse is also true. Goku would have taken a lot of time to awaken the full potential of Zamasu's body. Meaning that, even though Present Zamasu is theoretically post-BoG SS2 level, in the hands of Goku his body wouldn't be at that level yet. Just like how Goku's body in the hands of Zamasu couldn't even turn Super Saiyan at the beginning.

In other words, Zamasu killed Goku that easily because A) Goku was completely disoriented and confused B) He simply had no clue about what to do with that new body of his, he had all of his defences lowered (remember what happened to SSB Goku when he lowered his guard? He got oneshot by a laser beam). Meanwhile Zamasu went there with killing intent.

[Yuji]

Goku likely didn't know how to tap into Zamasu's power just like Ginyu and Zamasu didn't know how to tap into Goku's.

[Polyphase Avatron]

Goku likely didn't know how to tap into Zamasu's power just like Ginyu and Zamasu didn't know how to tap into Goku's.

Exactly. He had no experience using Kaio type ki.

[Hellspawn28]

Black's timeline was later erase, so we will never know what happen afterwards. I would imagine Goku in Zamasu's body would find a way to contact the other Gods about this in that timeline.

[Brogeta]

I would rather pretend the Goku black arc didn't exist.

[Cipher]

In addition to the situational elements mentioned above, I'm also going to offer that it's possible he was simply strong enough to do it right off the bat, even without transforming.

Black's respective forms are each a great deal stronger than Goku's, by the time we meet him in both versions.

EDIT -- I'm also going to offer that this probably should have been an "In-Universe" topic.

[super michael]

In addition to the situational elements mentioned above, I'm also going to offer that it's possible he was simply strong enough to do it right off the bat, even without transforming.

Black's respective forms are each a great deal stronger than Goku's, by the time we meet him in both versions.

EDIT -- I'm also going to offer that this probably should have been an "In-Universe" topic.

So if we want to talk in-universe and leave anything from real life out of it, we just create threads in the in-universe?

[Cipher]

So if we want to talk in-universe and leave anything from real life out of it, we just create threads in the in-universe?

That's the idea behind the board separation, yes.