Artists are almost always weird. To be honest I hate most "art" people. One would almost have to be mental to try and make a job out of art. Antisocial, awkward, depressed, imaginative, and many other related words easily describe the art world.
I think that one of the reasons for this is the imagination. Sometimes I have trouble putting imagination to paper. I could see how anyone would get upset or frustrated. Darger had some trouble with drawing so he did things like trace to get his story across.
There are many ways to look at Dargers life. Perhaps his reclusive behavior was due to his childhood. Or it could just be because he is an artist and they can be weird. I understand that certain things can be traumatizing, but people always read too much into stuff and create their own meaning when in real life he probably just a grumpy old man that wanted to write a story.
I also wouldn't consider it a great work of art. It is a waste of time analyzing all these people that consider themselves artists when I could be make my own. In the movie we were supposedly brought into the mind of Darger when in fact nobody knew him at all. That is what is really the waste. The people that were telling us about him didn't know him.