The superiority of the tiefling race over the goblin race still needs to be proven, in any case. There were people here who wanted to play for the goblin race, at least I saw a couple of people
I don't think that this has anything to do with superiority but the fact that the Tieflings have a human origin, from Humans making pacts with demons. In the game they are referred to as half human or half demon. When it comes to Goblins they are seen usually as evil with no human origin but a completely different race. I can see Larian keeping up with that stereotype of Goblins being evil, which is why you can kill them.