What exactly happened to Don Elkins?

[isis]

Book V talks on this subject. In Don's mind, he took Carla's position of being the brunt of the attacks from negative entities. He eventually became extremely paranoid and later physically sick.

He was asked to go to a hospital, which he refused. The police used tear gas (?!) to get him to come out of the house, at which point Don took his own life with a gun.

It's a sad story, and yet, as Ens Entium states, the circumstances surrounding his life were far more interesting.

"The color of his eyes even changed from deep, brilliant sky-blue to navy."

that is very interesting.