I'm not sure which genocide you are refering to, the attempted one against the Humans, or the actual one against the Monsters, so I'll argue both.



For the Humans, it was pretty much them or the Monsters. Many people believed that it was pointless for teh Monsters to even try breaking out, cause if they did the Humans would just kill them all. That, combined with Chara's less than stellar opinion of Humanity, meant that cohabitation was not see as an option. So either they could let Monsterkind be forever trapped in the Underground, ever having less and less space to live in, or they could do something, free their new family and everyone else from the prison the evil Humans put them in after slaughtering a large number of them. Plus, who knows if they even intended full on genocide, might have stopped at the one village, having claimed land for the monsters and use the gathered power to make sure no-one even thinks about attacking.



For the monsters, Chara didn't do that. In an often overlooked line, they straight up say that at hte beginning they had no idea what's going on and why they weren't dead, and that you taught them the purpose of their reincarnation. They learned it because of what you did, which was genocide, killing everyone you met, including their adopted mom. So yeah, they were dragged through pretty much hell and came up worse for it.