You can never really know someone completely. 

It's just another reminder on what you see outside might not be the same inside.

Even after knowing the truth in the end, I find it hard to sympathise with them. I mean, after seeing hundreds of pages of them doing (implied, because it's not really shown. You see bits of it from multiple PoVs) nasty things to other people... I just can't. It did help to make sense of their motives. At least that part of the mystery is solved. Kind of. 

The ending is unfortunate but fitting...? Kind of feeling a bit unsatisfying but since this felt more like a crime fiction instead of mystery, I'll say it's unfortunate instead. Honestly, I don't know what I hope to happen to them. Somehow, deep inside, I kind of knew it will be that way. It was shown again and again from the beginning. That person always got their way.