Genocide is a very strong word. It's specifically against another ethnic group or culture. You'd have to have a unique situation for that. Like, Nazi Germany. Moreover, you'd need a concentrated effort. Government oversight, a police force to enforce it, a controlled press to conceal the reality. As well as a way to dispose of the bodies in a safe manner so that the "normal" people, the perpetrators, can continue to live. If you just toss the bodies in the river, you're going to poison what you view as the "good" people. Illness infects everyone equally.

Now... if this was true anarchy where everyone was just bashing each other's heads open for some reason, you wouldn't need any of that.