Loved it start to finish. Now a clarification. I am mostly writing this to try and give my take on something some reviewers seem confused about.
We, the audience, are witnessing events as the main character perceives them. This encompasses everything we see, including! The videotape and reaction from his friend. The "creature" is undeniably an expression of his own darkness. Think "The Babadook" but with rage and loneliness instead of grief and mental illness.
The first obvious admission of this that the film makes is when the dying man in a biohazard suit (probably a tenant or realtor in reality) points at Ethan when asked "what is it?" From this scene on, there are numerous things revealing Ethan as the real perpetrator of all this violence.
Ethan installed the security cameras at his brother's home, thus he would know exactly where to stand to snag his niece without showing up on camera. His decision to break into his friend's home and kill him before he could be questioned is only logical if he could have been revealed as the actual killer if the vigilantes got to him first. Ethan's obsession with that board game, something he had attached extreme emotional value to that his brother just dropped on the floor, dismissively. Down to the final scene where he burns down his childhood home, "killing" the creature, or rather deadening his need for violence to achieve what his isolated and warped mind needed - his brother to look up to him again. Does anyone think for one second that an actual creature of that sort would just let itself be burned to death without a fight? It's not real, people!
And it is SUCH a better movie for it! Not a creature feature, but a psychological thriller. I thought it was fantastic.