I think that interesting is the key, rather tham likable.
Peope are facinated by killing because it's generally repellent and something that people struggle to do (I remember reading some research that 85% of soldiers aim high, either on purpose or subconsiously simply because taking another life is so difficult).
Because it's such an alien concept to most it's becomes interesting and we want to understand these people.
This is why probably the most talked about figure in WW2 is Hitler and not Churchill, Stalin or Roosevelt.