how does hucks practical joke on jim humanize huck?

It humanizes him because he almost kills him; sometimes jokes aren't so funny!

Huck tries to play a trick on Jim by leaving a curled up dead rattlesnake under Jim's blanket. But when Jim crawls into the bed he gets bitten in the ankle by the snake's mate. Huck kills the mate and sheepishly carries both snakes far away from the cave, embarrassed by the results of his behavior. Jim takes the jug of Pap's whiskey and drinks himself into a drunken stupor to avoid feeling the pain of his swollen leg. It takes Jim four entire days to recover from the bite and Huck vows to never touch a snakeskin with his hands again.