It was his Job. His Job put her in danger and tore her apart. He prayed they wouldn't be too late. Hayley was too young to die and Jack didn't deserve to lose his mother, especially not so young. He prayed he'd get there on time, hoped for it. Tried to believe he would.
Chances are he wouldn't. Which is part of why he was so upset driving there. He knew he would be too late. She would be dead before he made it. Before he could stop Foyet.
He was tearing up on the drive there, something he rarely did. But that just showed how much it hurt. How much he felt responsible for the coming death of his ex-wife and the mother of Jack. His 4 year old son.
Although they fought, he still loved her and she still loved him. His job just got in the way. As usual. And however much he thought that one day they would get back together he now knew that would never happen and it hurt. God did it hurt.
She would be gone. He had no hope.
That was confirmed when the gunshot was heard. Hotch broke down even more, tried to get to his old home as soon as possible. He had to catch Foyet and he had to save Jack.
He couldn't live with himself if those two things didn't happen.