It's difficult to craft a Utilitarian argument for stealing his wallet. The only easy way to do so would be if the money went to charity.

That being said, I would probably still do so. As a rationalist, I know it's not a positive action, but it would still give me (irrational) emotional enjoyment. Plus, you get a great story out of it. Imagine being able to tell your friends that you ruined Hitler's evening.