At first his violence was all defensive, at the store he refused to leave so the guy grabbed his baseball bat, D-Fens grabbed the bat from him before he could use it. Then the gangsters pulled the knife on him and he overtook them with the bat. It's like he just kept upgrading his weapons on his journey, from the bat to the knife to the bag of fully automatic weapons. It seems like the bag of guns really had an effect on him and played a major part in pushing him over the edge at the end(I believe Pendergrast was right he was going to kill his family and then himself).
Another pivotal point of the story similar to the Korean refusing to break the dollar(even though he had a shit load of quarters and the cash register was open) was the exchange with the homeless guy. Something I never paid much attention to before was that D-Fens thought for a minute about which bag to leave behind, like he almost thought about leaving the bag of guns behind for a split second, and if he would have it would have changed the whole course of his day. Instead he gives the guy his brief case with his lunch in it.