Even at seven I was disturbed by Tom Sawyer's failure to grapple with the reality of Jim's situation in the final chapters of the book. He sets up an elaborate plan to abuse the hospitality of his aunt and uncle, setting Huck's and Jim's lives at risk as well as his own, gets wounded in the process, and all for the sake of a rescue which he alone knows to be completely unnecessary. As a child I felt he got away with it rather too lightly and indeed is unfairly rewarded with Huck's continued admiration and a minimum of chastisement from his family, and my views haven't changed in 36 years.
Of course I'm even more disturbed as a mature reader by Twain's own failure to really rise above the racial stereotyping of his day. Jim is an instrument of Huck's own growing up rather than a three-dimensional character himself, and Mark Twain resorts to racist humour rather too often for comfort.
But it still remains a very enjoyable read, much better than Tom Sawyer (I think; though it is decades since I read the earlier book).