It is entirely on topic. The point is that we can kill goblin kids but not tiefling kids. Why should that be the case? The logic of why that may be so is innately tied into DnD concept of evil as it applies to goblins. I donâ€™t know how much more â€œon pointâ€ it can be.
In any case, I think the discussion is interesting and I am learning alot about DnD alignment system (personally, as a newb).
I also think the notion is of creating a pure evil race as something you can kill without regret (something Tolkien was adamant was a key part of fantasy writing and the history of the genre) is also an interesting side topic that naturally has come out of this.