I haven't read all the replies, so please forgive me if this has been mentioned.
Here's how I look at it. Think about spending life in prison. Excellent chance at getting raped. No freedom. No walks in the park on beautiful days. Institutionalized food. Screaming maniacs and more immature behavior by some inmates than you ever could have imagined. No chance at true intimacy with a spouse. You're treated like a child, having to ask permission to do the basic things that many of us take for granted. Someone is always guarding you, because you're an animal in a cage. If you're half-way sane, you have to live thinking about the violent crime you've committed, for the rest of your life. You'll never travel on holiday. You'll never spend another holiday in the living room with your family. If you have children, you'll never be a real part of their lives.
Or think about your life being very quickly ended with an injection or a jolt of electricity. Sure, probably some pain, but in a few moments, you're in a perfect world where there is no pain.
Which would you choose? I'd choose death, no doubt! Thus, it's much more of a punishment to rot in prison for the rest of one's life than it is to very quickly and humanely end your life (which almost assuredly did not happen to the people you victimized).
So, setting morals aside for a moment, I think it's a greater punishment to put someone in prison for the rest of his life instead of ending his suffering on earth!
