My player didn't want to play my grimdark setting

So I had a game go tits up when two of my players ran out on my game, and I dunno where I went wrong. I don't have time to get into detail but I wanna paraphrase it here.

My campaign took place in a grim setting (which is very well written, if i do say so myself), and it had gotten to the point where the party met a goblin army that wanted to use elven, human, and goblin DNA to create a master species.

So one of the players was playing a half elf and I immediately knew this was perfect to show the grim nature of my campaign! So being the talented author i am, I gave her character the first hand look at the hobgoblins and their need maintain genetic supremacy and their knowhow.

But apparently, the players got upset and even accused me of being a nazi! Which, by and large, is untrue; I have never said anything about promoting the superiority of any human race! I was only making a system in a fantasy setting that described a race of beings that wanted to create a master species and destroy all that were inferior!

Anyway, maybe I did it wrong? AITA?