“The mosque was located in a predominantly Sunni area.” And so it may be that the perpetrator was Shia. Or it may be that he was Sunni, but considered the Muslims in the mosque to be, for whatever reason, insufficiently Muslim. For that, as for so many other things in Islamic law, the penalty is death.

“At least 12 killed in suicide attack on Abu Ghraib mosque,” RT, June 28, 2016 (thanks to David):