Wrong. They didn't know he was unarmed. They did know that if he was wired with a bomb, he could set it off unless they killed him--not just wounding him (he would still set it off), but killing him. Experts who've worked around terrorists a lot in places like Israel know that the only way to stop a suicide bomber is to shoot him in the head, and do it more than once.
Then if he was really a suicide bomber, he would have set it off then and there, not RUN!