Gollum is a thief and a sneak, as well as a murderer (though I don't think Sam knows that part) and that's all that Sam can see, he has not felt the power of the ring, so he can't sympathise with Gollum the way Frodo can and the way audiences can. He sees Gollum as a danger to Frodo and their quest and that is why he's a bit mean, to show Gollum that he is suspicious of him. Also, the fact that hobbits may be weaker than Aragorn etc. is the whole point, that is why they may succeed, because they are what sauron will least expect and as Galadriel says "even the smallest person may change the course of the future.
P.S. I love Sean Astin, I think he's great as Sam, and Sam is the loveliest person who ever lived!