People say this is about a disturbed man, but it's more about the reality some end up living in society. If you've ever been completely alone, hungry and unloved since birth you'll understand this movie. Obviously this isn't for everyone. It touches on places we avoid, and for good reason.
It's completely realistic for the most part. I think the parts of murderous fantasy and the end relationship were more for plot rather than realism. Very heavy on monologues. The monologues can be really powerful and honest but they can also be a little repetitive.
Some of the messages in this movie are about class, morality, law, consciousness, and the meaning of life itself. Whilst the honesty and intellegence in the messages make them brilliant, it still becomes a little repetitive as nothing new is layered each time they're revisited. So this can feel slow.