I disagree with the artical about when it says that he is "not mentally ill we are believed to believe". It is clear in Many places that he is in fact mentally ill. Both characters talk about how they struggled with mental illness BEFORE there problems happened. Tiffany mentioned that she "wasn't every in the mood for sex" and felt "she had enough trouble taking care of herself" both signs of depression. The main guy also mentions life before. Also, anxiety disorders and OCD are mentioned in the movie that are again left untreated.
They were made to seem "crazy" but human. That is the whole catch in the movie, the beautiful balance between what is human about people that struggle with mental illness. It is stigma and idiocy if someone believes them to be "normal" people struggling. Yes, he had reason to respond in that sense, but key is that the average person wouldn't do such a thing. It's when their response go above and beyond normal that it is clear an untreated illness is happening. Often events just trigger things to come out enough to happen. He often has irrational actions in the movie that clearly mark his illness.
If there is an issue with the fact he doesn't like meds all statements about the meds in the movie include side effects that do happen. For some people meds don't always work. The greatest example would be John Forbes Nash. The worst movie ever for mental health drug companies would be "A Beautiful Mind" and that was based on a true story.
Just my thoughts.
Lillyleaf