Damien: “Life is ours to choose.”
Dark: “Why do we need to choose in life?”
The similarities in word choice are very, very telling. Also, Damien rather obviously turns gray after he walks away from Mark’s covered body. A small amount of people were saying it was the lighting, but no – that was an editing choice. That was a thing we were meant to see.
I’m pretty convinced that he’s Dark, based on the evidence we’ve got so far. I’ll be very surprised if he isn’t.