One clear example of both cruelty and irrationality comes in Chapter 15, when Huck, for no immediately apparent reason, decides to lie to Jim about losing one another in the fog:
"Well, this is too many for me, Jim. I hain't seen no fog, nor no islands, nor no troubles, nor nothing. I been setting here talking with you all night till you went to sleep about ten minutes ago, and I reckon I done the same. You couldn't a got drunk in that time, so of course you've been dreaming."
"Dad fetch it, how is I gwyne to dream all dat in ten minutes?"
"Well, hang it all, you did dream it, because there didn't any of it happen."
This goes on, seemingly for Huck's own entertainment, until Jim actually believes him. When he sees the leaves and "rubbish" on the raft, and the smashed ore, he understands it as evidence of the lie, and Jim's feelings are hurt. He was vulnerable, admits crying for Huck's possible lost life, and after all that, Huck just tries to make a fool of him for sport.
It was fifteen minutes before I could work myself up to go and humble myself to a nigger; but I done it, and I warn't ever sorry for it afterwards, neither. I didn't do him no more mean tricks, and I wouldn't done that one if I'd a knowed it would make him feel that way (Chapter 15).
Huck's behavior in this scene is both irrational and cruel, which even he realizes about himself at the end.