@SovietWeeb For the first question, her mind was already broken during the event where her father didn't acknowledge her and her mother. After her mother's death, and the fact that her corpse was simply thrown away, it was twisted even further. Instead of just being indifferent or pitying her, the other slaves actually giggled and even provoked her. Well, if you think more about it, it should only be natural that the other slaves don't treat her like a fellow human being, cause it appears feeling disgusted of beastmen is a common trait shared by most, if not all, humans. So surely they must have looked at her like an abomination during her time being caged there as well, and maybe even occasionally provoking her whenever they're bored (slaves don't have anything else to do in their cages).
As for the second question... This isn't exactly a spoiler in my opinion, cause these details was written at Chapter 2 of the WN, but ended up being left out from the manga for whatever reason (well, manga can't really do info dump I guess).
So basically, the hero only took roughly 3 years to kill the Demon Lord after he was summoned. Then another full year of being hunted by the world. The church's saints announced that he was an enemy, the kingdom confirmed it, then even all the crimes committed behind the scenes were blamed upon him. So his hatred was already being solidified over and over for a whole year. The manga sort of made it look like he was backstabbed only right after killing the Demon Lord, but that wasn't actually the case.
The reason he finally got captured after the whole year was also due to the princess. She didn't make it obvious that she took part in turning the hero into an enemy after defeating the Demon Lord. She only showed up later on after he was already exhausted, pretending to still be his friend and lured him into an inescapable trap. Afterwards,.. you get the scene where she made fun of the hero and claimed that she never treated another human from another world as a fellow human being in the first place, and it should have been obvious that she never considered him an ally.
tl;dr, As compared to the WN, the manga adaptation kinda sucked. The arts is its only saving grace. Even then, I still find the LN covers better than this.