is tom inexcusably crual? or is he just being a 13 year old boy?

I think yes, Tom does have a streak of cruelty. Even if we take the context of the time into consideration, it is still pretty nasty of Tom not to reveal thins info. Huck certainly cares much more about Jim's welfare than most white people. If anything Tom has become a symbol of the white status quo rather than a rebellious adventurer. The trouble is that Tom knows exactly what he is doing even though he is only 13.