[chapter 42] in this chapter, how does twain explain tom's earlier willingness to aid in Jim's escape?

Tom wakes and gleefully details how they set Jim free. Horrified to learn that Jim is now in chains, Tom explains that Miss Watson died two months ago and that her will stipulated that Jim should be set free. The old woman regretted ever having considered selling Jim down the river. Just then, Aunt Polly walks into the room. She has come to Arkansas from St. Petersburg after receiving a letter from Sally mentioning that Sid Sawyer—Tom’s alias—had arrived with “Tom”—who was actually Huck. Tom has been intercepting communications between the sisters, and Polly has been forced to appear in person to sort out the confusion.

Tom knew all along that Jim was free. He just wanted to have an adventure, and he does it at Jim's expense with the spiders, eating bedpost shavings, etc. He treats Jim terribly simply because he can. He does it for fun. Note the contrast between his desire to "free" Jim and Huck's desire--to help a man who has been like a father to him. This is the "last straw" for Huck that makes him decide to go away and not be connected to "society" any more.