Dunp them in Antarctica. "Vole on Ice".
I've long advocated dumping child rapists, murderers and other repeat violent felons on one of the barren rocks in the Aleutians with a knife and a blanket. They will either kill one another off, or learn to help one another survive. Nature ("mother" nature and their own nature) will give them what they deserve.
I'd be perfectly willing to drop the death penalty if their punishment was that sort of banishment.