Because the rules make it so you aren't the ones creating genocide. Sure, just turn off our emotions, wipe out everyone who isn't like you. Or feed off hate, see it as a thing of beauty and that destruction is the aim. That sounds like a brilliant plan. Maybe they did it because they were afraid and wanted to be stronger. But in going against the rules as you put it, in wiping out those who have wronged you or others, you become the thing you're fighting against. Maybe it starts off unnoticeably and it happens without you seeing it. Maybe the change is instant. But it still happens.
The good in this world? Even if it is something as simple as having friends and people who care about you? That's what makes us human. That's what we hold on to in order not to become consumed in the darkness and need for extermination or hate. No. It doesn't make it go away but it keeps us grounded and able to find other solutions.