Huh. No love for Yoko?
Anyway, to your spoiler bubble: They actually raise this point in the series (episode 26, when he's talking to Lordgenome's Head), and the Anti-Spiral King doesn't answer. While you could say that it's just they got a bit squeamish at the idea of universal genocide of intelligent life, I think it's more related to their overall tactics. Since they know that Spiral Power runs on raw determination, their entire strategy is an exercise in trying to break their enemies spirits as much as possible. So I've always thought that they decided not to wipe out life because, ultimately, they couldn't, or at least couldn't safely. Mop the floor with them and let them limp back to their homeworlds, you get broken, spiritless people who won't rise up again for generations. Try to wipe them all out, and once people realize it's win or die, they get increasingly more desperate and determined. Trying to wipe out all intelligent life in the universe could easily lead to the survivors generating exponentially more spiral power and bringing about the spiral nemesis just fighting back. Nevermind of course it was pretty much the entire rest of the universe they were fighting against; trying to wipe out an entire species just gives all the other ones that much more time to figure out how to beat you.
Also note that, by all evidence, Lordgenome basically set up the entire Humanity Anhilation Moon Drop scenario himself, or had some hand in doing so. If we suppose that this was a common happening for most races... Given that the Anti-Spirals were just one planet, convincing the great warriors of various planets to brutally oppress their people themselves is just that much more efficent.
This incidentally is one of the things I love about the show, it can actually hold up under shocking amounts of analysis, rare for the genre.