I just finished watching the movie a couple hours ago, it's after midnight, and this review might be gibberish, but I'll put it out there for you to judge. I don't think there are any spoilers here, but I think it's a better review if you've seen it already.
If Timothy Treadwell hadn't been killed by a bear, an end which he all but engineered deliberately, he would have self-destructed in some other way. In the footage he shot of himself, he comes across as diagnosably mentally ill--bipolar and paranoid, alternating self-hatred with delusions of grandeur. He rescued himself from alcoholism only to throw himself into a haze of sentimental attachment to animals, especially grizzly bears. The footage of bear behavior that he shot is an astonishing document of animal behavior as well as a simply stunning collection of images. When he gets in front of the camera, he tries to be a combination David Attenborough and Edward Abbey, a naturalist and an activist proclaiming that his very existence, his "study," is helping the bears. The truth is that he confirms the worst stereotypes of environmentalists--that they are self-righteous flakes--and worse, he habituated the bears to human presence, making them more likely to attack someone, which would inevitably lead to more bears being killed.
While Treadwell sees the animal world as pure and perfect, Herzog sees murderous indifference. Neither man has a complete and realistic view of nature. Treadwell's romanticization of it got him killed. Herzog's film shows nature without the creation of new life--the only baby animals shown are a cannibalized bear cub and a fox kit killed by wolves. Since his source material is Treadwell's own videotapes, his only possible narrative leads inexorably to death.
Herzog's direction allows the story to unfold, for the character of Timothy Treadwell to reveal itself gradually. We may not like him, but we are fascinated by him. Unfortunately, Herzog's narration, while it fills in important story details, is occasionally intrusive. He goes too far in directing us how to feel about Treadwell and his life, like a new age Michael Moore. He should have stepped aside more often and let the remarkable material before us, along with his deft editing, do the work of conveying meaning.
Like Mark Rittner in The Wild Parrots of Telegraph Hill, Timothy Treadwell found meaning for his life by communing with nature. Rittner and Treadwell both appear to be eccentrics, singing to the animals, telling them with desperation how much they love and need them. But Rittner is able to turn and walk away from the parrots when he knows that it's best for them, even if it breaks his heart. Treadwell couldn't leave when it was the right thing to do, and he got himself and his girlfriend killed, which led to the death of the bear that killed them.