Huck is trying to tell us that people can be good or bad, and you have to get to know the individual people before you can decide which.
I think the best way to answer your question of what Twain is trying to show us about people is to discuss the themes of the book.
People should do what they think is right, even if others say it is wrong.
One of the most important messages of the story is that you need to do the right thing. This does not necessarily mean following the law. Some people follow their conscience instead. Huck knows that slavery is illegal, and he grapples with the concept through the story. When Huck has to decide to turn Jim in or not, he struggles with it. His decision?
“All right, then, I'll go to hell.” (chapter 31)
He decides that if helping Jim escape is sinful he might as well go to Hell.
Treatment of individuals should be based who they are, not what they are.
Throughout the book, Huck also grapples with the idea that some people are bad while others are good, based on their actions and not their race. He realizes that not all adults are good, and that adults can be liars, cheats and murders that are not to be trusted. His father is a perfect example. Huck knows that his father is not a good person, even though he’s white and an adult.
That is nothing. I never seen anybody but lied one time or another. (chapter 1)
By beginning the book by talking about lies, Huck (and Twain) sets up this idea of lies and the truth. People need to be judged by their actions.