These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Soldier/Solomon David Nier. Lost his family and his mind on Pearl Harbor. He tried to join up the day after the attack, but was judged mentally unfit for duty. Eventually Solly decided to just go fight Nazis on his own because of something his brother said and flew to Poland. He killed key Nazi figures with the help of an informant who's identity he never discovered (Spy). He even killed Hitler!.
Spy agreed to work for the Nazis in exchange for sparing his village and family from being dragged off to the concentration camps. After months of spying for them, he found out that the Nazis had occupied his village, and his mother had been raped repeatedly, dying from her injuries. After killing every Nazi there, he was prepared to kill himself. He found out about Soldier's rampage in Europe, however, and began secretly feeding him information in an attempt to atone. And to make it worse, in chapter 15 is implied that he had an abusive father. In Chapter 20, it's confirmed. To the point where when his mother gave him a knife for a gift on his seventh birthday, he broke her arm and tried to take the knife for whatever purpose he had in mind. He had to hide from him and stab him in the back in order to survive the experience.
Scout had his home broken into by a French-Canadian with a knife and a rainbow shirt who was threatening Scout's mother and was planning on raping her right in front of Scout(a little kid at the time) when he came down. He even threatened to kill Scout. Scout ended up taking a baseball bat and beating the man to death after he continued to try and rape his mother. The experience ended up traumatizing him from that day onward.
Engineer's wife took custody of their daughter. Irene turned out to have done it to punish Dell and teach him a lesson about 'ruthlessness'. Dell had been against selling his Sentry design to the military to make money, which offended her Gold Digger sensibilities. Not only that, Irene didn't care about their daughter and was only doing it to spite Dell. And now BLU and RED Engineers had been mind raped over the course of a very lonely 2000 years on a deserted planet and 1000 years in an Australium mine, respectively.
Medic is a self-proclaimed "sadist of the vurst sort"... and depressed, the maskys call him "The Dead One". In chapter 13, he mentions that something broke inside him years ago and that his past is horrific, and in chapter 15 the RED Medic said that he "deserves Hell more zan any of ze others". And he almost killed his ex-wife the first time they had sex,and he is afraid that he could ever do something like that to someone else.