A short story that I read many years ago, possibly in an SF anthology, has stayed with me. I've forgotten who's the author, and don't necessarily agree with his

A short story that I read many years ago, possibly in an SF anthology, has stayed with me. I've forgotten who's the author, and don't necessarily agree with his point, but it is a good parable. Briefly, a man comes to the house of a famous scientist, who happens to have a mentally disabled son. The unknown visitor comes to plead, politely, for the scientist to cease working on a super-weapon project. He is heard, but is rejected. The visitor accepts it well, and asks to use the bathroom before leaving. After he has left, the scientist finds out that the man has given a gun to his son. He manages to get hold of the weapon without mishap, and wonders what kind of man would give a loaded gun to a disabled child ...