i dunno man...maybe im just a sicko....but if I had control of the whole situation id like to play with their minds a bit first, then introduce a bit of torture (not really do the torture, but make sure they see the equipment so that they get scared)...then stick them in a cell surrounded by waste (like shit), then play more with their minds, do a bit more torture (again no real torture but just to scare them), then find out how they like the consequences of their actions. Once they reply with tears down their cheeks and beg for mercy, id say "too late buddy, ur in here for life". If they ask whether or not they will be tortured, Id just reply: "yes, but you wont be told when - we will just come and drag you one day/night/morning"....
Hanging or lethal injection means the criminal wins (in a matter of speaking). They will be scared - but it will only be for like 5 - 10 minutes before the actual sentence is carried out.
there are some countries where the prison officers target the prisoners nails - so like snapping off the whole nail.......thats way too much. I reckon, just scare them a bit, thats punishment enough.