Lethal injection for sure! After committing mass genocide I have a feeling there would be some angry people out there looking for blood. If I was blind, paralyzed or in a prison its inevitable that someone was going to kill me soon enough, and their way of doing it might not be so pleasant as getting an injection lol. (yes I know lethal injection is basically the equivalent of acid being poured through your veins, but angry people can likely come up with much more painful and slow ways of killing me)
Edit: My mistake, just saw that I cannot die unless of old age if I chose prison.