Answer:
i know c!purpled definitely became unpopular for what he did to c!slimecicle but i'm not gonna lie, i think he was justified in doing it, even if he was wrong.
it's been a while since c!purpled was first scouted and employed to las nevadas, so i feel the need to mention that the way he had initially been involved was through c!quackity blowing up his ufo, one of the few things on the server that indicated his presence where he otherwise felt forgotten.
c!purpled is a lonely character. there's camaraderie to be found between the other characters on the server, but never him and someone else before he was recruited by c!quackity. he shows this loneliness in the way dogchamp accompanies him wherever he goes, despite the constant danger on the server.
where some people would hide their pets, c!purpled keeps his by his side, otherwise who else would be there?
one of the main things that c!quackity said in the stream where he blew up the ufo was that "the ufo was your legacy, and i've taken it from you."
legacy is so important in this server. when you leave a mark on the smp, it's remembered, for better or for worse. for c!purpled, who was alone and resented that, his ufo being blown up, which was a reflection of him, was something that cut deep. c!quackity and his ruthlessness in destroying it and knowing what he was doing would not be easily forgotten by c!purpled, no matter how much he could claim it was water under the bridge.
the way c!quackity framed this deal between the two of them was that c!purpled had the choice to be forgotten with his ufo or remembered with las nevadas. there was no way he could walk away happy in either instance, but there was a way he could walk away remembered, which c!quackity knew was the only thing that would get him to join.
c!purpled needs to be remembered because he needs to feel less alone. being remembered means he's important which means people will be there for him.
the ufo was an extension of himself, his legacy, and c!quackity took it away without giving a it a second glance, because at this point in time, he didn't care! he didn't care for attachments, his or others, and that made it easier for c!quackity to destroy them.
by the time c!quackity began to relearn the fact that attachments were natural and he would be burdened with them no matter what and c!slimecicle was one of them, c!purpled had been letting this bitterness fester. he had been pushed aside for c!slimecicle, in his viewpoint, and c!quackity was someone who promised him greatness and belonging and hadn't delivered.
it didn't matter that c!quackity meant for them to be together, that he had acknowledged his fondness for c!slimecicle, that las nevadas mattered more to him than just material gain, because c!purpled didn't know and to him, it looked as though c!quackity destroyed his legacy only to pass las nevadas on to c!slimecicle as thought it didn't matter.
and then we get to the betrayal!
on the surface, it doesn't look like c!purpled would have any reason to go after the revival book. he has no connection to it, nothing that would make him desire it because there's no one he needs to be revived.
for c!purpled, however, the revival book isn't something he needs for the revival aspect, but rather the fact that it's something that c!quackity has that he values. he wants to hurt c!quackity the same way c!quackity hurt him, and the most obvious way he knows to do this is to take away the one thing that he had been working tirelessly for, for months on end.
the revival book is less a revival book and more of another part of c!quackity's legacy.
its why he brings up c!quackity destroying his ufo in the middle of asking about the revival book, otherwise why else would he mention it? they're connected, both extensions of the person behind them. c!purpled and his ufo, c!quackity and his revival book.
yet at this point, c!slimecicle is there and c!quackity clearly cares for him, and c!purpled notices, so what else to do but hurt c!quackity the way he was and take this extension of him away?
because in a way, c!slimecicle was an extension of c!quackity— his humanity. everything about c!quackity and this web hes created centres around humanity and legacy and people and connections. in forging a connection with c!slimecicle and a legacy in las nevadas, c!purpled was caught in the crossfire.
c!purpled, in the end, was lonely and desired company, had his legacy destroyed by c!quackity only to be pushed away again by him after promises of riches and togetherness, and lashed out the only way he knew how because he was a mercenary and never offered the same support c!quackity was.
i can't fault him for that, can i?