[H]is whereabouts and actions in the months leading up to Mr. Thompson’s murder have largely remained a mystery. Some family and friends have said they were unable to reach him starting shortly after he returned from his backpacking trip to Asia, a trip that now appears to have been pivotal for him. […] Interviews with fellow travelers and local residents, along with a review of Mr. Mangione’s writings and communications, suggest a shift from seeking human connection and community to isolating himself, and becoming increasingly preoccupied with how to make a statement about injustice.