What to do about cat behavior problems?

My cat has always been a bit..odd, but lately his behavior has been worrying. He’ll jump up on a person’s lap, but if they move or talk he’ll growl at them and hiss, seeming very angry. He occasionally gets very scared and vocal around people, hissing and running away, and, again, acting angry. We’re worried that the behavior may escalate into aggression, since he’s so odd and unpredictable. Nobody who’s met him would describe him as “nice.” He just acts crazy.
We adopted him as a kitten, and have taken proper care of him. Why does he act like this towards people?