I would strongly recomend:
the memory of a killer- its a belgian film about a old hitman who is in the begining stages of alzheimers.
boondock saints- here we have two brothers trying to rid boston of all sorts of criminals. A lot of fun to watch and really enjoyable
Le Samourai-this film just rules, there's really no other way of putting it. Alain Delon is the prototypical hitman in each and every way
sword of doom - okay, so yes this is a samurai film, but it fits the hitman category too. This is definetly one of the coolest films that you will see.