'Be careful with that thing. That gun's got a curse on it.'
I've watched the movie quite a few times, trying to verify for sure, but...
Wade warns Tucker about the gun being cursed. Subsequently, other than Wade himself, every single character who touches the gun dies - the only exception is the man he hands the gun to on the train. However, once he's on the train he whistles his horse over to make his escape, so it's quite possible Wade ended up killing that guy as well.