Goblins are "usually Neutral Evil". What this means is up to the DM, but in my experience is generally taken to mean that, while they can of course be other alignments (perhaps if raised by humans or something) their "default" in this setting is Evil. In other words, killing them is OK as long as you don't have reason to suspect they're Good, but actual genocide is frowned upon. Remember, these are adventurers, killing monsters and taking their stuff is part of the job description.