Let us remember that Huck's father was hardly an excellent role model for Huck. He was a drunkard and a liar and a cheat. He is also physically violent towards Huck. Interestingly, however, Huck doesn't seem to mind too much about these negatives and does say that he enjoys living with his father in comparison to all the rules and regulations of living with the Widow Watson. If you want a good reason for why Huck ran away, consider this example of his father's behaviour from Chapter Six when his father drinks himself into a frenzy:
By and by he rolled out and jumped up on his feet looking wild, and he see me and went for me. He chased me round and round the place with a clasp-knife, calling me the Angel and Death, and saying he would kill me, and then I couldn't come for him no more. I begged, and told him I was only Huck; but he laughed such a screechy laugh, and roared and cussed, and kept on chasing me up.
With such behaviour and regular beatings, it is hardly a surprise that Huck decides that he wants to run away and thus plots to fake his own death so that he can have the freedom to live his life the way that he wants to.