you know that basic question people ask: if you could go back in time, would you kill Hitler? Here's a line of thinking along with that ...
Eirik
says
what if a lot of the killers in death row are actually saving us from future tyrants and dictators?
**crikey** 
most of them haven't killed anyone before they take power, though didn't musselini stab a kid at school?
Obsolete
says
This is a dangerous line of thought. Would you justify pre-emptive imprisonment/death a la Minority Report?