Yet Another Interpretation

I just saw this for this first time and wanted to put my interpretation of it up here. Hopefully I'll get some replies from fans about whether they agree or disagree, and I think this'll include too many spoilers for a review.

Mark has a wife with whom he is beginning to fall out of love. There are hints of infidelity from both of them. They have a young son.

Mark is in the employ of a shady group of characters for whom he is some kind of liaison with demonic forces. This is a very dangerous, maybe illegal job, as we see Mark is paid in briefcases full of cash. Citing the demands of family life, Mark refuses to take another contract with them.

Either in retaliation for his refusal to continue working for them, or because he somehow pissed off whoever he dealt with as part of that contract, these demonic forces invaded the mind of Mark's wife Anna and turned her into the worst kind of crazy bitch, taking her existing bad traits (lying, manipulation, emotional abuse, hysteria) and cranking them up to their maximum.

Drawing energy from the emotional abuse she inflicts on her husband, her husband's friend with whom she had been having an affair (Heinrich), and the two private investigators she murders, the possessed Anna summons a demonic force which slowly takes physical shape.

The demonic forces Mark was messing with used the existing infidelity and immorality of these characters as a gateway through which to manifest in the physical world, furthering their agenda to basically destroy humanity.

At the end, the pink-socked guy Mark was hired to do business with shows up and is all smiles as Mark and his wife lay dying, as if it's all a game to him, a game he's been playing all his life and will continue to play for years yet.