So I just learned that inert gas asphyxiation with nitrogen is now an approved method of execution/death penalty in Oklahoma (US). So a murderer/rapist can have a state funded and administered effective and peaceful death, but I can't??
Just some nitpicking first: While rape under certain circumstances is still on the books as a capital crime in some states, like Oklahoma, it generally hasn't been something you get the death penalty for – you need to do a murder. And since 2008, it's moot, as it's been ruled unconstitutional by the Supreme Court.
But anyway, the method of execution is *supposed* to be effective and peaceful. Only death is on the books, not torture. That's why the electric chair and gas chamber went away in favor of lethal injection, and why even lethal injection has been heading on its way out. They've all been credibly challenged as not painless and peaceful.
And keep in mind that most people don't want to die. So while it seems like a cruel irony to you, being put to death is considered an awful punishment by most of those subjected to it.