I feel like even if people here made him go crazy and insane however, he might be in denial from the first place. In military flashbacks he seems to discover his insanity too. 

Maybe they played role in him realize that he is indeed insane, maybe they didn't have a much impact on him going crazy, maybe he was just insane from the beginning and he just didn't realize it yet.

Moonjo might have saw it in him and decide to let him realize the way he is as crazy as him.

I personally feel that Jongwoo was on the brink of insanity…..like he was still tethering onto that one last string of humanity before he came to the apartment. He also was much more closer to his gf before shifting into the apartment, and once there the people around him started making him so crazy that when he realized his gf also might leave him, he finally snapped and went on like full crazy