On the other hand, since his father was following sharia, why would that drive him to jihad instead of American values?
We have no disagreement here. Yours is a fair counter to the “violent childhood” defense. Except that a court or jury would have to accept the premise that sharia was responsible for the childhood. We’re in a society where all violence is the same, and the cause is irrelevant, unless the “violence” is caused by a white Christian conservative. Otherwise, we’re not allowed to accept that sharia is violent; don’t want to offend the peaceful religionists.