You haven't shown that. You keep talking about this creature that was animated by its Evil creator God with an unwavering desire to do Evil in the world, but you haven't shown any substantial evidence that goblins are actually this way in D&D, or that they aren't the Giant's way in D&D (consider the many similarities between The Dark One and Maglubiyet, for example, even if they aren't an EXACT correspondence--which is fine, since sourcebooks state D&D pantheons may serve as a starting point for DMs to develop their own).
Meanwhile, you've neglected to address the numerous examples of Usually X creatures behaving in a Not-X manner.
According to you, you've already gone through the trouble of developing a backstory for goblins that gives the players a logical consistent justification for considering goblins uniformly Evil...which is fine.
On the other hand, considering goblins uniformly Evil so that you don't HAVE to come up with their backstory would be lazy at best.
If you've been doing the former rather than the latter, there's no reason to be defensive.