many complain that this is a video game but at least this way she can take revenge on those who bothered her and save the world simultaneously.
And considering how much motivation she needed to kill a single human she needs at least this level of justification to commit genocide instead of committing suicide.
Plus there's also the possibility that the admin/hacker/AI/goddess (or whatever she is) is lying.
Maybe this is one of those situations where if you die in the game you die in the real world or this isn't even a game at all.
If it is a real world, it is possible that the protagonist is the reincarnation of the Demon King and an evil goddess is tricking her into awakening her Demon King powers.