How is civilization portrayed as barbaric in Huck Finn?

Civilization is portrayed as barbaric in Huckleberry Finn because it condones slavery. Huck feels full of guilt because he is helping Jim to escape. His Southern society has taught him that slaves are property rather than human beings. The friendship he develops with Jim traveling on the Mississippi teaches him, however, that Jim is a human being. Huck can't help but remember all the times Jim has been loyal to him and sacrificed for him. So Huck wages a war of conscience. 
In the upside-down moral universe in which has been brought up, Huck has been taught that everything he is doing for Jim is a sin. He "knows" he should write a letter revealing Jim's whereabouts. Yet he decides that, even if it means he is going to go to hell, he can't. We as readers perceive that Huck's moral self-torture is wrong. Huck is, in fact, doing the right thing in helping Jim.
It's also worth also noting that Huck and Jim encounter people who could be considered barbaric. For example, the King and the Duke put money ahead of other people's welfare. They are quite willing to defraud orphans. Twain exposes a world in which too often people put things ahead of people, instead of the opposite.