Vin Diesel would suck in Hitman - which would make a far better movie than Half Life. To play 47, you need someone very cold, calm, calculating, and detached. Vin Diesel is far too "in your face." Jason Stathom would be a far better choice.
But as I said earlier, I'm against the whole video game turned into a movie practice - it ruins the game, or if you are going from a movie to a game, the movie. For one, I loved the Godfather - and now EA has a Godfather game under development. I also heard that they might turn Resevoir Dogs into a game - thats just sad. How can you make a good game out of a botched diamond heist? I mean seriously, do you want to play knowing that you are going to lose?