I was reading this question and I noticed Daniel Roseman's comment:

Point of clarification: Smeagol wasn't "hobbit-like", he was actually a hobbit. "I guess he was of hobbit-kind" means that he was of the same "kind" - ie race - as the hobbits that Frodo knew.

and further down Jon Purdy's comment:

Specifically, he was a Stoor, a variety of Hobbit that had an affinity with men—as opposed to a Harfoot or Fallohide, who shared characteristics with dwarves and elves, respectively.

So Sméagol was actually a Hobbit, just like Frodo and Sam. But where it takes quite some time before Frodo starts showing some signs of greed and corruption caused by the ring, it only takes a look at the ring for Sméagol to go completely mental and kill his brother. Sam has also seen the ring lots of times, and even kept it for a while, but he didn't try to choke Frodo during their trip.

Why did the ring have such a different effect on Sméagol?

Edit: Now that I'm thinking about it, Bilbo went a bit off the rails near the end, but he never actually hurt someone to protect his ring either.