Allow me to enlighten you.

Death has always made people stronger, now just imagine that as a
constant, to forever be at the peak of your power. As one of the living
dead, you feel as if you cannot be harmed. And it's logical. If you're
already dead, how can you be hurt further? Of course, this is not the
case, but it allows you to cut past your fear of being hurt for a few
moments, at least until the scimitar strikes your chest.

Nothing really does matter beyond power. Beliefs. Morals. These are all
things we throw around just so that we can claim to be more powerful
than others. Sure, people will claim otherwise, but that's just
ignorance. We all know it's true.

Being cut off from the natural cycle is just a change of pace, so to
speak. The same routine every day gets boring after awhile, does it not?

As for willingness to give up everything for something temporal at best:
All power comes at a price.

To feel your own fleshing decaying is a wonderful thing, really.
Initiate Sephiroth, Advocate of the Unjust

Penned by my hand on the 11th of Ultio, in the year 419 AD.