A good reason. There is no good reason for Genocide. I wouldn't wish it on anyone. No civilization deserves that, no matter how evil. Because there are innocent civilians among the evil wrong-doers.
I could condone it depending on the situation... acceptable losses and all... I'm not evil just realistic... but then again I don't live in a fictional universe like SW where everything can be black or white and where morality is an absolute nor do I wield the force... lol