When you really get to thinking about it, there is quite a bit.  It's an unfortunate thing, really.

Take the entire character of Pap.  He was abusive, beating and whipping Huck on a regular basis.  He also took Huck out of school, because he wouldn't have Huck "put on airs over his own father and let on to be better'n what he is."  So, out of pride he stops Huck's education, takes him into the woods and locks him in a cabin all day long.  He tries to steal all of Huck's money too.

Then, consider slavery.  In and of itself that was cruel, but Miss Watson was going to sell Jim, separating him from his entire family.  Huck plays numerous cruel and mean pranks on Jim thoughout their journey, which shows just how ignorant and below him he thought Jim was.  Then, at the end, Tom and Huck make Jim go through all sorts of painful and humiliating acts at the Phelps farm before they let him escape, all for their own fun.

There are other examples throughout the book also.  The duke and king almost get tarred and feathered numerous times.  In the deep south, an angry mob chases a drunk with all intents of hanging him.  The Shepherdson/Grangerford feud leaves many people dead, including Huck's friend Buck.

So, I hope that these examples helped!  Good luck!