^If he had a degenerative brain disease affecting his thinking and judgment, then there would be no reason to suspect foul play.
It also wasn't the first time that he displayed paranoia of being followed-
http://www.mlive.com/news/grand-rapids/index.ssf/2013/07/documents_detail_ex-gvsu_quart.html
"Jennifer Finnerty told investigators that it wasn't the first time her husband, Cullen Finnerty, had a "paranoid" episode. Eighteen months earlier, she said, he drove 150 miles to Grand Rapids from Detroit because he feared the FBI was following him."
Instead of driving home from Detroit a year-and-a-half earlier, he took off for Grand Rapids in western Michigan due to fears the FBI would follow him, she said. According to Jennifer Finnerty, her husband remained in a state of panic for four to five days.
Cullen Finnerty had a past addiction to painkillers, said his wife, who believed a pill he was given by an acquaintance may have caused the paranoia that spurred his trip to Grand Rapids."
So he had a past experience where he took painkillers and had a paranoid experience that lasted several days. This outdoors trip sounds no different:
"Jennifer Finnerty told police her husband called her around the same time.
Cullen Finnerty called out, "Hey, are you there?" three times, she said. He said he was talking to "that guy" and believed someone was 20 feet behind him.
She heard rustling noises on the other end of the line and asked her husband what he was doing. Finnerty told his wife that he was taking off his clothes. She told him to stay put, and the call was disconnected.
Jennifer Finnerty said she then sent her husband a text message that instructed him not to move, because her brother and father were coming to pick him up."
^^More paranoid experiences after taking painkillers. I don't think it's any coincidence. This guy had a brain injury and combined with painkillers made him go off his rocker.