Huck's crisis could be summed up as his natural sense of decency and morality colliding with conventional southern moral teachings. Conventional morality, buttressed as Huck understands it, by the Christian church's doctrine, tells him he will go to hell for wronging Miss Watson by helping her slave escape. He is guilty of abetting theft. Jim is nothing more than a piece of property. Huck realizes he has violated the trust of the white community.
However, as he ponders sending the letter that would betray Jim, his feelings for Jim overcome him, and his loyalty to and love for another human being who has been good to him trumps the hollow morality of southern racism. Huck thinks:
And got to thinking over our trip down the river; and I see Jim before me all the time: in the day and in the night-time, sometimes moonlight, sometimes storms, and we a-floating along, talking and singing and laughing. But somehow I couldn't seem to strike no places to harden me against him, but only the other kind. I'd see him standing my watch on top of his'n, 'stead of calling me, so I could go on sleeping; and see him how glad he was when I come back out of the fog; and when I come to him again in the swamp, up there where the feud was; and such-like times; and would always call me honey, and pet me and do everything he could think of for me, and how good he always was; and at last I struck the time I saved him by telling the men we had small-pox aboard, and he was so grateful, and said I was the best friend old Jim ever had in the world, and the only one he's got now ...
This is an extraordinary moment for Huck as he embraces his own moral compass. He consults his own conscience and his own humanity to determine his own moral path. His deeply felt experience that Jim is fully human overcomes society's lesson that Jim is nothing but a thing. He tears up the letter because he can't deny what his heart tells him: that Jim is worth more than a false moral code, that Jim's freedom, autonomy and dignity as a human being is far more important than any loss of so-called property Miss Watson might suffer. Huck learns that all humans have intrinsic worth, no matter what their skin color.
It's worth noting that Twain frames this powerful moment with his characteristic irony. The reader recognizes that Huck has developed a moral maturity and capacity for empathy and feeling that few adults in his society have achieved. However, in the upside-down moral universe Huck inhabits, he believes his wholly good action will send him to hell as a sinner.