In the anime Monster, there is a character who always smiles, but could not express any other emotion.
He watched a super hero cartoon, which influenced him as a child and made him a saviour. He kills people when he thinks he can save people but always blacks out and it is always bloody as hell.
May I ask what is his name?
You're looking for Wolfgang Grimmer. (Can be found in this list of characters.)
First seen in chapter 78, Grimmer is a freelancing journalist researching Kinderheim 511 who is drawn into the search for Johan when he helps Tenma. Nearly always smiling, he is an apparently friendly, polite man who is good with children.
Just a note for the future: I found his name just by searching "monster anime smiling character" on Google.
