It can also be not entirely or to any degree selfish. Depending on the particular state one is in, the world can be like a place where one is only causing harm, no good. Friends and family suffer because of oneself, one is betraying work and society and one is also constantly disappointing the once beloved pet. It really can be an act of "whelp if everyone is better of without me, then...", it can be an altruistic thought in a really twisted way that stems from a highly disturbed sense of selfworth. I have been there, and it was not all about me, I was genuinely convinced I would make the world a worse place in every way, that every person loving me is fooled etc.