I pity them.

I think you misunderstood it. If you carefully look, Reiner looks sad. And he covers up in the end, his family too is confused. He made them look human, unlike their beliefs. And there was a part where he forgets about being a warrior, right. Meaning he liked them, in my opinion he says hellish, because he may get killed.

Exactly. I mean, who would really use a girl eating a potato in boot camp as 'hellish'? That's sounds like something a person in the French aristocracy would use if a peasant ate a large piece of bread.

I can see why you would say that and althought that could be one reason why he said hellish but i agree with the op. Right before the transformation into the armored titan, reiner says "if i never knew these kinds of people lived inside the walls then i wouldve never become such a half-assed piece of shit". Its hellish because all his life hes been brainwashed into believeing that all eldians within the walls are demons, and hes been raised with the sole purpose of eradicqting them and ensurung victory for the people of Marley. Upon living amongst them, however, he realised that they were nothing like the demons reiner expected. His 3 years within the walls completely contradicts everything hes known beforehand, hence the split personality. For Reiner, its hellish knowing that despite the fact that hes lived with his enemies, that was the one time he ever enjoyed true companionship (other than bertolt and annie maybe) for the rest of his life. I personally think the said expression is because hes reflecting on the fact that all of his friends within the walls are his enemies now. Things can never go back to the way they were...