How does the return of Tom to the story in these last chapters result in a shift of tone and mood?

The tone changes from Hucks altruistic concern for his friend to Tom's contrived motivations for Jim's release. The tone is more cynical. Huck develops his own value system as the story progresses. Near the end, Huck truly wants to help Jim. Huck feels that Jim is no longer an escaped slave but a human being and friend, more descent than any white person he has known. Tom sees Jim as a an opportunity for adventure. Freeing him would not necessarily be the right thing to do but rather a rebellious act worthy of his intelligence. Perhaps Tom entertains a vague notion that Jim might deserve freedom but it is certainly not as developed as Huck. Tom's re-entry into the story shows just how much Huck has evolved and how much Tom has stayed the same.