I think that subconsciously, Ted sees Peggy as the trophy in his battle with Don. She was Don's protege. Having her symbolizes Ted's ultimate victory over Don. It's just that, once Don is out, the spoils are not so sweet. Not Peggy per se, but the whole sordid business that Peggy has come to represent to Ted. He did after all encourage Don to take that drink before he made his chocolate pitch. Screwing your enemy's protege is one thing. Kicking a sick and struggling man off the cart is another.