He loved her. Jack doesn't think he ever loved Laura this much, he should. She was the mother of his children and his wife. Debbie, what was Debbie to him? No one. A subordinate. Someone he shouldn't feel this way for.
But he does. Enough to even let Tom Chandler walk away from all his crimes. Mickey couldn't and wouldn't. His love for Kate Spears driving him to want to see Chandler in ruin, broken, and shamed. He understood it.
Jack can't stop it. Can't protect Debbie from any of the worse of it. He can't protect her from Chandler either.