Dark, creepy, unsettling are words that come to mind. I just think it should be made clear this isn't a villain /hero movie. I would not recommend this for children.
It is the sadly disturbing beginnings of the Joker. The Heath Ledger Joker was carefree and light hearted compared to how this one feels. I admit, I do not follow the comic books themselves. It is so well acted it's uncomfortable to watch. Personally I thought it moved slow but I am a sensitive person so every step of development was painful to watch. I didn't think the Joker had happy beginnings but I thought somehow there would be a distance, tragic but unreal. Fantasy. This movie is very real. If that is what you are looking for you will not be disappointed. So real it hurts.
I repeat. Not a typical hero/villain movie and NOT suitable for children.