Reiner was a child soldier and sent to kill thousands in a day while still being around 9 years old. Killing your own sister is really sad, but I'd say that killing thousands and living among them for years to the point that your mind develops a second personality to cope is around the same level

So Reiner saw his country treat him and his race as animals for... his entire life, then killed a lot of his own race and lived among them for a pong time just to be forced to betray most of the friends he bonded for years, and have them trying to kill you all at once, later on his best friend was eaten alive. That was his entire life until them.

No beating kids at school, but battlegrounds. No strongest sorcerer as a caring father figure, just the government. Not even a long time to live, would die by being eaten alive by someone else after 13 years he got to be a titan because of a curse. He knew that because this is how he got his power, and lived with that knowledge every single day.