wrong. a shy london teenager before he left for syria and became the masked murderer featured in the gruesome videos from isis. now britain's prime minister is vowing to crackdown on the extremist ideology that enables groups like isis. >> here's my message to any young person here in britain thinking of going out there. you won't be some valued member of a movement. you are cannon fodder for them. they will use you. if you're a boy, they will brainwash you, strap bombs to your body and blow you up. if you are a girl they will enslave and abuse you. >> reporter: hundreds of british men and women are believed to have joined the terror group and others like it in syria and iraq. cameron is vowing to crack down on spewers of hate and bolstering groups to speak out against isis.