I've seen this show a few times and have followed Mr. Treadwell's 'adventures' since I first heard about him in 2001. I found the most interesting part of the Discovery piece was the last half hour or so when his friends were sitting around talking about their impressions of the movie. They really seemed to drive home the point that Tim would have loved the movie because it made him look like the "rockstar he always wanted to be". IMO his real purpose of spending all that time with the bears was that he wanted to be famous and he loved the adrenaline rush it brought him.
I also believe that when he went back that 13th year that he knew he wasn't going to leave Alaska alive. He and Amy actually went to the airport to fly home, but Tim got in an argument with an airline employee about the validity of his ticket. It was then that he decided to go back to the 'Grizzly Maze' relatively late in the season and I don't think he ever planned to leave again. He and Amy had been fighting because she had to be back in California to start a new job. I think Tim expected her to leave.
Some people have suggested that Tim knew the bear that ate him and Amy, but I don't think he did. That late in the year, new bears had moved into the area and Tim's 'friends' had either moved out or headed to their dens. It was really interesting watching the last footage Tim shot of the bear that ate him. For some reason he seemed more fascinated with that particular bear and he stopped narrating about the bear's actions. For some reason he also shot footage of Amy in the same scene as the bear, something he avoided previously when Amy was with him. I think he knew that bear was capable of eating him and he was willing to let it happen. I think he planned on Amy getting away and telling the story of his death. Unfortunately he didn't anticipate Amy sticking around long enough to be eaten or he thought the bear would be satisfied with him.