He already witnessed many worlds like Planet Babari, that was just the last straw. The mortal who defeated him, Goku, did so on sacred ground and acted in an extremely disrespectful way towards a God. It doesn't take a genius to figure out that a GOD, a future SUPREME KAI, should be trated with respect, not touched so casually and molested.Skar wrote:I viewed Zamasu as a parody of the Fallen Angel trope. He seemed like an angsty teenager who was mad at the world and decided to commit mass genocide across the multiverse after encountering one barbaric mortal race and being defeated by Goku, a mortal who surpassed him. I think part of the joke was how quick his descent was and how he rushed to enact his plan.
Was he aware that all the Gods of Destruction were basically the same as Goku and mortals who trained to reach that power? He rushed to switch bodies with Goku without considering if there were mortals out there stronger than him including candidates for the GoD position which DBS proved in later sagas. Imagine if did kill Trunks and then ran into future Broly, Jiren, Toppo, etc. They could easily take out Goku Black and then Zamasu would just have to hope that his immortality could wear them down.
Even if he never encountered a mortal more powerful than Goku, Zamasu was defensiveness against Zeno so his plan was basically kill all the Kaioshin to take out the GoDs and then wipe out all mortals hoping Zeno never finds out. The Universe Survival saga reveals that Zeno or Grand Priest are able to scan an entire universe and give a mortal ranking which means they definitely would've noticed when a universe's ranking dropped down to zero. Without even considering that, Zeno or Grand Priest should at some point notice that all the gods below them are dead and wonder what happened to all of them. Some might say Zamasu won when Zeno wiped out Trunks' timeline but I recall his goal was to only kill mortals and preserve their planets so no one really got what they wanted with this outcome.
Your point about Broly, Jiren, and Toppo is honestly just a plot hole, because Toriyama hadn't yet created those characters when the Zamasu arc aired, and there was still the old evil Broly. Maybe Toppo was already the Destroyer in the future and died when Zamasu killed the Universe 11's Supreme Kai. Maybe Jiren died due to a terrible illness. A lot of things can happen in 20 years. Jiren might be strong but he is still a mortal. Ultimate strength won't help you against a virus.
Zeno and the Grand Priest don't really give a shit about mortals, they were happily erasing trillions of innocent mortal lives to make a point. Maybe the Grand Priest would be angry that Zamasu "killed" his children, but then again as the manga explains Zamasu was doing his best to keep hidden from the King of All, which obviously wasn't easy, but it was possible. The entire Project Zero Mortals took place without Zeno having any clue. I don't even know if there is a Future Grand Priest, as it was literally never even mentioned in either the manga or the anime, so maybe the Grand Priest somehow trascends time itself.
Besides, Zeno is just a naive child, I have no doubt that Zamasu would be able to befriend him should it come to that. Zamasu easily fooled Goku and Gowasu into thinking that he was a good guy, he is cunning and deceitful.