And you just pointed out a flaw or loophole in your argument. You're right, the Brood kill on instinct and reflexively; as a species, that is how they survive. The slavers, killers, and other criminals cannot really be considered a species in and of themselves and act with malice, although I suppose some could argue that they're also acting on their own warped instincts and reflexes. Nonetheless, wiping out the Brood constitutes genocide; influencing the Brood to target and kill slavers, killers, and other criminals does not. Food for thought.
Frankly, this discussion could go on for days, and I hope it will!