Continuity mistake: At the beginning and end of the film, there is a narration who the subtitles label as Sitka, but at the very beginning, we see that the narrator is actually Denahi.
Continuity mistake: After Koda frees Kenai from the rope trap, the rope disappears twice from around his (Kenai's) leg: first when he is walking towards his brother Denahi (then the rope reappears around his leg as he runs away), and disappears again when he meets Koda again inside the glacier (only to reappear again when he snags it around a rock/ice chunk and trips).
Continuity mistake: In the scene where the bears are riding the mammoth, Koda is touching Kenai, but when the camera angle changes, they are at least a foot away from each other.
Other mistake: At the end of the film, as Kenai walks up to Koda after he's been transformed back into a bear, Koda leaps on him in joy. If you look closely at Kenai's hind legs, there are a few frames of animation missing. His legs go from a standing to a sitting position instantly. It's barely noticeable (no pun intended).
Factual error: During the opening scene when Kenai is running away from a herd of stampeding caribou, several of the caribou do not have antlers, even though both male and female caribou sport antlers. One of these "antlerless" caribou is even seen running near the front of the herd and is easy to see.
Answer: His ultimate goal was to protect his brothers from the bear. If he tries to attack her directly, there's a good chance she'll dodge the blow, strike back at him, and then go back to attacking the other two once he's too injured to defend them. Whereas the means for a glacial collapse were right nearby, and even if the fall didn't kill the bear, Sitka figured it would at least frighten her off.