It depends. In a case where it's flaming obvious I say give them an injection. For instance it costs $75,000 to lock up a madman like Martin Bryant every year - for what purpose?
I'd probably settle for voluntary capital punishment. Give people the option to die over a long term sentence. 10 days later they die, but they have a one week cooling off period after they make the decision.
I'd probably rather die than be buggered in jail for life.