Not only the arsenal but the training. Where did he learn to shot like that. Some one on TV said he shot with about 50% accuracy. And his booby-trapped apartment? It sounds like an elaborate set-up...
My guess on the arsenal is he just bought the stuff on credit card. He knew he would either be dead or in prison so he was not worried about paying it off.
As for the accuracy, in a packed theater, it would be hard not to hit someone with every shot.