In my personal opinion, Zamasu was by technicality, wrong, but, he has the point right.
You see, Zamasu was always rotten himself, believing himself as this righteous deity for the Gods when in actuality was only for his own appeasing act to eradicate mortals and seek higher than Gods. As mortals are rotten too, but the kind of rotten that's looked down upon in his eyes: savagery, bloodlust, mischief, mockery, uncivilized, and worst of all, mortal. To which in case, yea, mortals can be horrible and evil, Zamasu unknowingly relates to all of that. He's right about mortals against the Gods, but he has the wrong conclusion to balance that.
The funny thing is, Zamasu hates the very thing he has most common with. And wants to be rid of it, but doesn't want to correct himself because it his eyes it's the wrong thing to do for the right reasons. He belives Gods shouldn't be flawed, he shouldn't be flawed. He's full of himself, he's delusional and psychotic. He's aware he's evil, but isn't aware of either of the two. He's a hypocrite too, dares himself as superior to kill mortals as Goku Black but has the audacity to use a mortal body to do exactly that. And his invincible counterpart just makes it worse for his ego, and fusing together is literally the embodiment of illusion of justice. Double the delusion, double the evil he reserves.
Abusing his role as a Kai, a God, a beautiful divine being, looked up upon, into the wicked, misshapen, twisted, insecure, flawed, and broken man. In the end really was more "mortal" than there ever was before and after. And died in the worst version of the very thing he hated most: a monster.
Ah... the irony of a villain. Beautiful.
