Peter was fighting against himself as much as he was fighting her. He had her heart in his hand - a rough squeeze, and that would be it. But the things Xander was saying was staying that hand. He wanted to know that woman. The one he was talking about. That was someone worth saving.
He heard glass shattering, and knew there wasn't much time. If he teleported, he'd lose the telekinetic hold. Peter clenched his hand tighter - but not too tight - and used his other hand to grab her throat. With those holds in place, he threw down his arms, telekinesis hurtling the witch to the ground.
Then the magic struck him. The blast tore through him, tearing into his flesh and soul, drowning out the feel of his abilities and leaving him stunned and staggering, just in time for the glass to strike.
He could have recovered. Even with so much damage, even with his healing ability momentarily suspended by the raw magic, he could have survived. Had not one long shard of glass pierced his skull at the brainstem.
Peter crumpled to the ground, bleeding from a dozen wounds, eyes glazed and blank.