Could he get the death penalty for that?
I would assume he falls under US jurisdiction and will most likely be returned to the US and deemed unfit to trail due to medical grounds.
If this man is legitimately sick then the US military will have a responsibility to treat him correctly. This is another indication of the stress/pressure that these soldiers are under.
Whether he was sick or not or simply did this out of hatred needs to be investigated.
The only outcome this will have is even more hatred and support for the Taliban against ISAF.