Enchantress From the Stars/Nightmare Fuel

Despite being a YA book, it still has some very horrifying moments.

• The death of Ilura. She distracts the Imperials from the ship, but then one of them points his gun on her. Now people of The Federation have a built-in mental shield which could deflect the weapon... but the oath forces Ilura to drop the shield, basically cooperating in her own murder. The Imperials vape her into nothing, leaving not even ashes to bury.
  • The feeling of Elana when she later getts in a similar situation and, having never dropped her shield before, contemplates whether this will work or not - note that "success" may very well mean her death, while failure would likely bring total disgrace among her own people. Thankfully, she is "just" paralized, which leads to...
  • Elana (a 14 years old girl, by the way) searching for a way to surely die, because this is a better alternative. Her solution? Being crushed by rocks from the rock-chewer!
  • The (supposed) treatment of Elana after capture. The paralyser leaves her fully conschious, but unable to move any limb and thus completely at mercy of her captors. And then Jarell was supposed to change her into Imperial clothes while she was like this! In her case, he briefly allows her to move so she can change herself, but others are implied not to be so lucky.
• General Imperial treatment of natives. They are not just considered inferior, they are considered nonhumans at all and are treated like cattle. Most of them are semi-paralyzed all the time, for weeks.
  • Generally the idea that a spacefaring civilisation may be so barbaric. Thankfully, the Empire would not attack our world now, as they only prey on totally primitive cultures (nukes are considered big enough a deterrent), but what if there are even more advanced aggressive civilisations out there?
• The extraction machine. The idea of a device that can read your brain directly is... unsettling in many ways. The implications for the society (especially given that it is not only used on "locals") are even more fearsome. Think Nineteen Eighty-Four Up to Eleven!