I don't think Zamasu's character was butchered. Hell, I think that he's not only the best written villain in all of Dragon Ball, but the best written overall character in all of Dragon Ball.
And I've gone into great detail about why I have that stance in the past:
[spoiler]People often complain that Zamasu had no real backstory, but I disagree with that. While he was shown to be a Kaioshin, Zamasu cared greatly enough for the development and prosperity of the universe, that Zamasu often questioned the worth of mortals and didn't believe in their ability to handle conflict as they were prone to commence war in a repeated cycle. And given the events that transpired in the main story, he has a very valid point. The Kaioshin of Universe 7 stood by and watched Freeza terrorise most of the galaxy for God knows how long before Goku and Future Trunks took care of him. It's made even worse by the fact the Kaioshin were strong enough to defeat Freeza with one blow, but they still stood by and did nothing. Then you take into consideration that the events of the Android/Cell arc and the Majin Boo arc happened on purely through the arrogance and selfishness of the main cast. He also did not agree with how the Kaioshin would not be more directly involved in mortals' actions like the Gods of Destruction. Even in the manga, his scoffs at the idea that mortals can be trusted to handle important matters, let alone rival the might of gods.
The moment with him him and Gowasu travelling to the Babarians world was huge turning point in his character. Because if he ever needed more fuel to add to his argument of how dangerous and unruly mortals could be, that was it. Zamasu claimed that they should destroy the planet because they will never learn to be civilised, to which Gowasu is shocked by this response and in an attempt to prove him wrong, Gowasu and Zamasu travel 1000 years to the future, only to find out that the civilisation has not advanced from the small culture it originally was, and the race as whole still remained hostile, angry and aggressive race, as the same two of the Babarians are seen fighting. And just to add the cherry on top, one of the Babarians tries to attack Zamasu and Gowasu at first glance.
Meeting Goku was where his character officially went off the deep end. After meeting Goku however and losing to him quickly in a sparring match, his views radically changed and his distrustful nature towards mortals only grew. Annoyed at Goku's carefree attitude and lack of respect, his distaste for the gods not quelling the dangerous nature of mortals evolved to a radicalism-level. Shocked at how a mortal like Goku could gain a power rivalling gods, let alone even obtaining such power, and so recklessly challenge a God and defeat him easily, Zamasu came to sternly believe that mortal life by nature is an evil.
Goku Black and Future Zamasu is the embodiment of Zamasu basically giving into the his dark side and goes through with his plan to be a more active Kaioshin and try set right what he thinks has been done wrong by other Kiaoshin standing back and letting mortal create all kinds of havoc. He is extremely sadistic, as well as savage in battle and also relishes the opportunity to test his new power or abilities, but also shown to be very pragmatic is his way of achieving his Zero Mortals Plan. And never wastes an opportunity to display his superiority complex and his plan to create a utopia by eliminating all mortals.
Merged Zamasu is basically the accumulation of all the arrogance and self-entitlement of Goku Black and Future Zamasu. He literally sees himself as the embodiment of justice and having delusions of grandeur. His belief that he is truly this supreme God of justice that will create an new utopia for the universe even drive him to tears, as his feeling of responsibility to set everything right that he thinks is wrong overwhelms him emotionally. Of course, once Vegetto and later Future Trunks prove to be too much of a match for him, Merged Zamasu is reduced to nothing more the equivalent of a deranged madman swinging around a axe and butcher's knife at the same time. Constantly screaming and attacking with more savagery and fury than before and grinning like a psychopath, while making outrageous declarations of Godhood. It was at that point that Zamasu's descent into darkness had gone full circle, as he had officially become just as destructive, violent, aggresive and unhinged as the mortals he vilified and believed the universe would be better without. Of course, the dramatic irony of the situation never became apparent to him, as Zamasu still saw himself as the saviour the universe needed and wanted. Even in his death, his immortal soul and conscious spread across the world, the universe and even throughout timelines. Showing that even with no physically body to carry out his deeds, his spiritual body still feels compelled to become one with order and justice. If there's one thing you can't deny Zamasu had, it was dedication to his cause.
I think some people don't have a proper grasp of Zamasu's goals or motives, because Zamasu is actually a very complicated character. At the start, you see a young, bashful and down-to-earth Kaioshin who ultimately calls a spade a spade and knows of just how dangerous mortals can be and openly questions the system that he is a part of. He feels, given the role he has, that he doesn't have the true freedom of doing enough to curb the continuing cycle of violence that is tarnishing the imagine of the Universe he is meant to protect and look over. Zamasu was clearing getting tired of being a bystander and actually wanted to be an active role as a Kaioshin and instead of just watching mayhem spontaneously unfold before him, and that is what makes his fall from grace all the more raw and amazing to watch. He had good intentions for the sake of his universe but the fashion of which he went about them became more extreme with how events would later unfold with him visiting the Babarians planet and encountering Goku. It's what makes him quite an awesome villain, and even to a degree, quite a tragic character.
In short, Zamasu is a fantastically written villain, and has many layers to him and which make him, in my humble opinion, the best written character the franchise has ever produced.[/spoiler]
