The worry is not necessarily that the systems become "alive", though, we are already bad enough ourselves as a species in terms of motivation so machines don't need to supply the murderous intent: at any given moment there are at least thousands if not millions of people on the planet that would love nothing more than be able to push a button an murder millions of other people in some outgroup. That's very obvious if you pay even a little bit of attention to any of the Israel/Palestine hatred going back and forth lately. [There are probably at least hundreds to thousands that are insane enough to want to destroy all of humanity if they could, for that matter...] If AI becomes powerful enough to make it easy for a small group to kill large numbers of people that they hate, we are probably all going to end up dead, because almost all of us belong to a group that someone wants to exterminate.
Killing people isn't a super difficult problem, so I don't think you really even need AGI to get to that sort of an outcome, TBH, which is why I think a lot of the worry is misplaced. I think the sort of control systems that we could pretty easily build with the LLMs of today could very competently execute genocides if they were paired with suitably advanced robotics, it's the latter that is lacking. But in any case, the concern is that having even stronger AI, especially once it reliably surpasses us in every way, makes it even easier to imagine an effectively unstoppable extermination campaign that runs on its own and couldn't be stopped even by the people who started it up.
I personally think that stronger AI is also the solution and we're already too far down the cat-and-mouse rabbithole to pause the game (which some e/acc people believe as the main reason they want to push forward faster and make sure a good AI is the first one to really achieve full domination), but that's a different discussion.