Death penalty.
The great problem though, is if the verdict is wrong just one time it negates the whole principle. Who tries the jury?.
Ludovic Kennedy wrote a fantastic and extremely disturbing essay based on that young lad who supposedly shot a policeman. The fact that the kid had severe learning difficulties just made it worse.........poor soul didn't even know he was going to be offed until the last moment.
There are people who within them have some capability which we would describe as evil, but does killing them under state sanction make us any less evil? Complex question which I sure there will never be an answer to.......