The snake slithering out of Cain's mouth was real, only it was really slithering through a cardboard tube beside his head.
The closeup vulture footage was achieved by planting roadkill atop an overlook by the Kentucky River, high enough for them to swoop by easily. A 400mm lens and cropping of the camera's 4k abilities down to 2k allowed the shooter to be far enough away to hide in the trees.
In the final shot of the film, "the Mark of Cain" is seen briefly as a sign on Cain's wrist when he adjusts his bag and walks away.