Chaosmancer
Legend
Genocide. Give me a context whereby Genocide is a 'good' act.
The Shy'liek are a race of intelligent beings that can only survive by constantly consuming all types of matter, especially the brain matter of sentient species, but any matter will suffice. If they do not consume matter, they die. If they do consume matter they grow larger, and need to consume more matter, in an endless cycle that eventually leaves nothing behind but a vaccuum, whereupon they will consume and kill themselves.
If not all of them are killed, either by starving them or killing them directly, then all life in the galaxy is destroyed, including themselves.
It takes some extremes for something like Genocide, but when dealing with fiction, you can find it eventually.