In chapter 100, Ahab meets another whaling captain who's lost a limb to Moby Dick. The other captain says his arm got snagged on one of his harpoon's barbs, and Moby Dick dragged him down beneath the waves. He would have drowned, had the harpoon barb not ripped a gash clear down the length of his arm and thus worked itself free. The gash was so severe that the ship's doctor had no choice but to amputate the arm. Yet, after relating this story to Captain Ahab, he implies that his arm is still inside Moby Dick much like Captain Ahab's leg, which the white whale bit off. Was this just a continuity error on Melville's part?
More likely the captain was being hyperbolic. He knew the whale hadn't actually swallowed his arm, but was using the phrase as a dramatic metaphor.
This is something I never quite got, is Moby-Dick supposed to be evil or is it just limited to Ahab's Rage Against the Heavens thing mentioned on the main page?
The ambiguity is entirely intentional on Melville's part. On the one hand, Ahab's crew is hunting whales, with the intent to kill them and harvest them for oil and meat, so Moby-Dick's attacking them is justifiable self-defense. On the other, there is something different about Moby-Dick, that makes him tougher and more aggressive than your typical sperm whale. How much of that is simply filtered through the eyes of a crew full of Unreliable Narrators and has been debated by Fine Lit scholars for the better part of a century. It's most likely less a case of Moby-Dick being straightforwardly "evil" and more of him being symbolic of untamed savage nature, against which Man has always struggled, and which Man will often anthropomorphise as an enemy.
There are a lot of scenes where Ishmael narrates things he really shouldn't have been there for, such as a private conversation between Ahab and Starbuck.