If we're going to simplify things down to good or bad, I'd say yes, Tom is a bad person for not telling anyone that Jim was already free. There are no excuses good enough to justify this behavior - or at least to qualify this behavior as good.
The arguments that Tom's behavior is defensible because he is just a boy or because he is a product of his society have to be rejected, in my view, as insufficient moral arguments which serve to condemn boyhood and early 1800's America rather than insulate Tom from moral condemnation.
I disapprove of Tom's actions, but I don't think he was intentionally being cruel. Tom is both incredibly selfish and a product of his society. He was just enjoying the adventure, and he never would have assisted in Jim's escape if Jim had not already been free.
The way I see it, Tom is just a boy on an adventure. He is at that developmental stage where he is not aware of his surroundings and what he does or does not do and how that effects people around him. He wanted the adventure and was not thinking (as most teenage boys don't) about how is actions would effect those involved.
That being said, I don't consider Tom a bad person but just being selfish and a typical boy of his age. He didn't do anything that terribly endangered anyone, didn't kill anybody, he just wanted to have fun and it was purely innocent.
Tom is a product of his society. He is a white male with a sense of entitlement and self-importance. Jim is a black man, and Huck is a poor white, and therefor of lesser worth to 1830s/40s Southern society. Jim and Huck's feelings don't matter as much according to what Tom has been taught, and Tom assumes a leadership role whenever he enters the scene. Huck and Jim do not really have a choice but to follow Tom's leadership, because of his social status and power in that society. While Tom should have, of course, told Jim that he was free, Tom is not made of the same moral fibers as Huck is. Tom is also very immature, and only concerned with himself. While Huck has had to grow up and mature due to his tough life, Tom is still stuck in childhood, and he is mainly concerned with his own self-fulfillment. It is a game to him, and he is determined to make most of the opportunity by complicating the game to get the most adventures out of it. This last section is an interesting juxtaposition to the rest of the book, though, as Huck and Jim have been having very real and very frightening adventures all along, but once Tom enters the story he is acting on fabricated adventures. Tom is really no better than the Duke and King, as he is a swindler and a fake, and using others for his own gain.
At several points in the story Huckleberry Finn, Huck comments on how Tom loved adventure, to the point of absurdity when Jim is incarcerated, but could have simply not eve escaped, but just left his confinement. Tom creates an elaborate scheme for Jim to live as a prisoner and keeps him involved in pointless tasks; this suggests Tom's willingness to propagate the concept of slavery. Certainly Tom isn't acting in everyone's best interest, but puts his own quest for adventure and the social standards of the day ahead of what would be best for Jim. He should have told Jim he was free at the outset