Without getting into semantics, whether the act of killing yourself is "self-prioritizing", or selfish, I do think it's selfish, strictly from the standpoint that it is being done to fulfill some kind of need for youself - to stop physical pain, mental anguish, get out of a dire circumstance, what have you. If your ctb benefits someone else somehow, that is secondary. The primary benefactor of ctb is yourself, and by definition, that makes it selfish.
Honestly, I don't see the big deal about whether it is selfish. We do things just for ourselves all the time. I don't think selfish in this regard should have the negative connotation that it does. Suicide is usually a "last resort" kind of thing, and if stopping years of pain, or anguish, after other avenues of resolving the issues didn't work out, is called selfish, then so be it.