"They were going to die. They knew that. Somehow, despite any effort they made, Riddle would find them and kill them to get to me. That fact was something they could use. My mum worked out how. She was as logical as you, see, and understood Riddle would get to them via a weakness in their circle. He would torture and kill all their friends one by one to find them. They started with the Longbottoms. Poor Frank and Alice ... left to the depravity of the Lestranges. It was Riddle's insurance policy, in case he'd chosen the wrong boy to go after.