Paris attacks: Omar Ismail Mostefai identified as assailant per BBC News

There's an odd line in this BBC report

Apparently Omar Ismail Mostefai 'is said to have regularly attended the mosque in Luce, close to Chartres.'

Hold on, if he regularly attended a mosque then why did he carry out this terrorist attack? 

If Islam is a/the 'religion of peace' then isn't that a little odd. Surely the more one attends a place of worship for a religion of peace the more peaceful one would become?