Pity is a ridiculously bad and irresponsible reason to let a monster live. Diana's job (at least in her warrior capacity) is to be a guardian of the innocent, not to feel good about herself. "Aww, it's not the poor thing's fault" is not much solace to Genocide's victims, past or potential, nor is it a moral justification for not doing everything in her power to prevent those deaths. "I would feel like it was vengeance and I don't want to be that kind of person" is an abhorrently selfish motive for permitting further atrocities at the hands of a demon. It's tantamount to stopping AIDS cure research because, after all, the virus never asked to be a killer, and now that your friend came down with it you're too angry and have too much of a personal stake to feel right about wiping it out.
"You slay the monster, Kal." Whether an evil is unwilling to change or incapable of change is irrelevant. The why does not matter. All that matters is the what; a thing that will not stop being evil, ever, must be destroyed.
I don't dispute that Ares is someone who could reasonably be presumed unwilling at this point in his career; I strongly dispute that any category of "will not stop being dangerous ever" could possibly include him and yet not include Genocide.
(Also, it kind of makes Diana look like a chump, and hard to take seriously, when she spends six issues repeating ad nauseum how badass and painful Genocide's death is going to be and then just goes "whatevs" and implausibly one-punches Ares instead. She used to actually have some credibility when she said "I will end you," and the thing with Max gave her a lot more. After this, though, I'd take her less seriously than Clark.)