Neal's never been truly scared of Peter before.
He's seen him royally pissed off, and had him threaten to send him back to prison, heard him say harsh words that he knew he didn't really mean - but he always knew, deep down, that Peter would never really do anything to hurt him.
Not until Neal really gave him a reason to.
"You son of a bitch!" Peter snarls, crossing the room in two strides, grasping Neal by the collar and slamming him hard against the front door, hard enough to take his breath. "He has my wife!"
"I-I know," Neal stammers, holding up his hands in a defensive gesture. He tries to meet Peter's eyes, but he can't quite make it. He knows that he deserves this. "I-I'll find him. We'll get her back, Peter, I swear..."
The breathtaking blow from Peter's fist falls across his face, driving the words from his lips, and dark spots of color dance before his eyes as Peter releases him, allowing him to fall to the floor, against the door.
"You'll do nothing," Peter spits out with contempt in his voice that stings worse than the blow, "but stay out of my way while I save Elizabeth's life. And if Keller lays a single hand on her, Neal, so help me..."
Peter is silent, and Neal chances a look up, to see him struggling visibly with his own rage, his fists clenched at his sides. He abruptly loses the battle, closing the distance between them and grasping the front of Neal's shirt, his fist slightly choking him as he leans in close, eyes hard and ruthless as Neal's ever seen them. His voice is a low growl deep in his throat, and Neal knows that he means his words with every ounce of will he possesses.
"... if she's hurt... you won't make it back to prison."
Peter releases him roughly, and Neal winces in pain - but it's the ache of loss in his heart that brings tears to his eyes, makes him close them before anyone can see. He gets to his feet and quietly makes his way out into the night - on his way to do what he can to fix this, to repair what he's broken - if he still can.