Suppose he tells you that he did it because his religion told him to. Then you look in his religious texts and lo and behold, his religion DOES tell him to do that. Suppose several more Muslims do it and you find that the vast majority of their fellows agree that they had every right to do it, and they are donating money to help them do it, and they celebrate every time another Muslim does it. THEN do you start thinking maybe their religion is not compatible with your Constitution?
The Judeo-Christian holy book sanctions violent, immoral actions, too.
Do you really want to get in a pissing match of dueling verses?
I'm not a fan of any religion. I'm also not a fan of persecuting individuals who have done no wrong.