I think the point of not killing Genocide, is that Diana took pity on the creature.
This. Especially when you factor in Genocide being created from Diana's corpse. I dunno, it's almost rather Frankenstein's monster-ish. It's done horrible things, yet it's pitiable at the same time because it never asked for this.
Whereas Ares seems to be the architect behind this whole mess, the one who has actively been trying to do harm and cause harm, and threatening to do more harm. If he can cause something like Genocide to be created, then he is the true villain here, it would seem.