The central conflict of this novel relates to Huck's crises of conscience.
Huck's main struggle in the book is with his conscience, the set of morals with which he has been raised.
This internal conflict leads Huck to reflect on his actions often in the novel, beginning early on when he decides to fake his own death to escape his father's abuse. When he has escaped to the island near town, Huck suffers from a guilty conscience as he watches the townspeople mourn his loss and search for his body.
It is in regards to Jim, however, that crisis and conflict of conscience is at its most poignant.
...Huck confronts the ethics he has learned from society that tell him Jim is only property and not a human being. By this moral code, his act of helping Jim to escape is a sin.
Huck debates with himself about the right course of action. Should he help Jim run away, knowing that this is a crime? Or should he turn Jim in, thereby betraying a friend? Later he must decide whether or not to help Jim escape or to abandon his friend to his fate.
This crisis can be seen as a conflict between Huck's moral instinct and his moral instruction. As this conflict determines the outcome of the novel's plot, it should be seen as the central conflict of the book.