Not killing them. Just ensuring that they can never die, but will forever live alone.

Failing that, kill everyone they have ever loved, presently love, and will ever love. That'll get 'em from the inside, and if they don't hate life so much that suicide is necessary, then you dice them alive. Obviously starting with the less important parts, and making sure they don't bleed to death before their time.

I am one sick bastard.