Apathetic mass-murder is unjustifiable, and the ultrarich would need to justify it to their families, who would then need to justify it to other people they know, and so on. And remember that we're talking about the majority of the global population, an event like that couldn't be hand-waved away like one would do for a minority group.
I consider DOGE actions to be pretty much exactly what you describe, with their sudden destruction of USAID and defunding of climate science
Iran just mass murdered their protesters earlier this year, and they needed to convince humans to pull the trigger. It didn't seem to be that hard for them to justify.
It's probably a lot easier to convince the military AIs we're building to do it.
I don't particularly like the situation, but I plan to do what I can to be on the winning side. The world is going to be what we're all in on building, and for people on the winning side it may be a utopia. Either way, I know which side of the gun I want to be standing behind.
Also note that an AI well aligned with the people who initially trained it wouldn't need to actually kill anyone that didn't take action against the owners, it would just need to outcompete people for resources, and let them fend for themselves in extreme poverty. There's no need to act one way or the other beyond that.