Why didnt Matthew...

You cannot claim self-defense against another if you yourself are committing a crime at the time, which Matthew Modine's character was. Simply his presence there in the building was illegal, after Carter had filed a restraining order against him, ordering him to keep away from the building. So technically, he was already committing a crime himself at that point, by violating the restraining order. Currently, he had no legal right to be there. Carter himself mentioned it: "But you're breaking the law, Drake." He would have been charged with murder if he had simply killed Carter. Remember Carter even planted a crowbar on Drake to set him up and thus prevent criminal charges against himself. Make it look like Drake tried to attack him with a weapon, along with his illegal presence.