What are you going to do when someone comes into your home, ties you up and attempts to kill you? When you escape do you think you are jsut going to waltz out of the apartment?
In this particular instance? That's exactly what I'd do. If he could break free of his restraints, make it to the kitchen, grab a hammer and come back to kill his attacker without getting shot then he obviously wasn't in as much danger as you think he was in. He would have been better off leaving the apartment and calling the police, holding someone at gunpoint is a serious enough crime that he'd probably be safe for at least several years.
I'm all for self defense, I keep a gun, a crossbow and several knives and swords where I can reach them if I ever need them and I wouldn't hesitate to use them against an armed attacker if I thought it was the only way to save my own life. That being said, if I can safely get away from my attacker (which was clearly the case here) I have absolutely no right to come back and kill him later.
I think it really depends on how much time elapsed between the time he broke free from his restraints and the time he hit his roommate/captor with the hammer. If it was a very short period then he could claim that he did not know that he could escape the apartment to safety.
If however several minutes passed then the self defense claim is more shaky as the several minutes would have given him more then enough time to escape