I think the point of not killing Genocide, is that Diana took pity on the creature. In a way, it's a prisonner of it's own nature; it was made to be pure evil, and was never given a choice in the matter. By showing it mercy, Diana is hoping it can change.
But Ares, in the meantime, has deliberately chosen to commit ever act of evil he has done, willingly, knowingly and gladly. Diana feels no such pity for him.