Hm, for those who don’t know the comic: A) It is brilliant, and B) it derives a lot of punchlines as well as actual plot from citing the D&D rules and roleplayer’s laws, so this is one case where discussing the rules in context with the work of fiction makes sense.

My first thought was: The whole plot started with a classic change of perspective, but that’s not all. As it’s been stated, the things we see the Goblinslayer, some of his guards, and the Dwarf Paladin do are evil from every angle – torturing Vorpal and, back in the times, Thaco like that, killing an innocent dwarf child which, according to the Dwarf Paladin, just spent too much quality time with the monsters (who took care of the kid, after all)…
We also see Goblins do bad things though, like enslaving Dies Horribly for example.

It is not just a matter of perspective, I think. The original adventurer party as seen in the first chapter were enemies, but they weren’t portraited as evil per se. I think they were even starting to question their views a little – the dwarf was, I think- because what they had seen didn’t fit their expectations of Goblin behaviour. I have to read that up again, though.

The whole plot lives off a problem one might get when trying really complex storytelling within the D&D world, and according to the rules. From sheer player perspective, even more so in the PC games, races like the Goblins, later Orcs, and so on are meant to be cannon fodder that provides EXP points. Good chars need EXP too, and so all those petty booty races must be evil. But the concept of having senient races that are entirely evil is problematic enough from today’s storytelling perspective.
It works in fairy tales, but if you spin modern thinking characters into an archaic fantasy setting… well. It doesn’t work out for either side like the books say it should. That’s what drives the whole story in this case.