The Daily Mail says he was possibly in the process of converting to islam. This looks more like he was the leader of a jihad cell to me.

Friends of Boyle’s previously said Boyle had an ‘academic interest’ in terrorism and may have been converting to Islam when he was captured. 

The bruise on the forehead, called a “Zebiba”. It is an ostentatious show of crypto-piety, caused by deliberately banging one’s forehead onto a hard floor while doing Islamic ritual, for no other reason than to cause a bruise in the skull so others will know you banged your head into the floor while doing Islamic ritual.

Somewhat like street prayers, but it leaves a more lasting mark.

That makes it more than a passing interest and a possible conversion, although yes, this photo may have been years later.

from Shoebat.com:

There are two major red flags that when taken together become a RED ALERT. On his forehead sits a zebiba, caused by friction generated by rubbing the forehead on the prayer mat multiple times per day during Muslim prayers.

 

The second cause for concern is the beard without a mustache. Taken separately, these traits may or may not reveal a Muslim fundamentalist.

 

However, when a Muslim has both, the odds of having a fundamentalist in your midst go up exponentially: