The major point in keeping these wretches alive imo, is that if we kill this lot, then we may kill other men (or women) who are innocent.
That for me, has always been the biggest practical reason to be against the death penalty. Everyone knows our prisons are full of innocent people. Sally Clarke and Donna Anderson would be dead now. Not to mention the Birmingham Six, the Guildford Four, etc. etc.
You can't have the death penalty for one bunch of scumbags and then not apply it in a miscarriage of justice. And for me, the risk of killing an innocent person outweighs the money it costs to keep vile people alive.