Why is Matt angry at himself at the end of Chapter 3 in The Sign of the Beaver?
Matt is angry at himself because he has allowed his father's rifle, with which he had been entrusted, to be stolen from him. He had had a hunch that Ben, the stranger who had invited himself to dinner and spent the night at his cabin was not to be trusted, but he had failed to act on his suspicion. Now both Ben and the precious gun are gone, and Matt has "no protection...and no way to get meat."
Ben, a slovenly-dressed, heavy-set man, had appeared at the cabin alone, seemingly out of nowhere. He is garrulous, asks Matt a lot of questions, and expresses a keen interest in the rifle hanging on the wall. Ben is evasive when asked about himself, but gives the impression that he is on the run from the law. The unwelcome visitor invites himself to have something to eat, telling Matt, "thought mebbe you'd ask me to stay for supper," and the boy, having always been taught to be "mannerly" towards his elders, serves him up a large portion, which he gobbles down greedily. Ben then asks for some tobacco, but Matt has none to give him. After talking awhile longer, Ben falls asleep.
Matt is not sure what to do about this strange man who is taking advantage of his hospitality. Although one part of him says he shouldn't begrudge him a meal and a place to stay, Ben's presumptuous manner makes him uneasy. Matt wonders if Ben is "dangerous - perhaps even a murderer," and thinks that "he'd be sensible to stay awake and on guard." He actually at one point thinks to take his most valuable possession, his father's rifle, and keep it close to him, just in case, but in the end, he does not take this precaution. Matt resolves "not to shut his eyes all night" so as to keep watch over the stranger, but he falls asleep nonethless. When he awakens, Ben and the rifle are gone (Chapter 3).