In Acts 2 & 3 when Luis Sera and Chopper Mike, the only people who actually help Leon (save for Ada), get killed horribly by Saddler.
(After Mike saves Leon again) Leon: Thanks. When we get out of here, the drinks are on me! Mike: Yeah! Hey, I know a good bar! Cue Saddler and his mook hitting him with an RPG)
While some people would find it Narmy, Leon constantly yelling out the names of dead people he hardly knows isn't that much of an overreaction. After fighting through an entire village to save Ashley, just for her to get kidnapped again, he has to go through an entire castle and defeat all the enemies there. Then Luis, one of his only allies in the game, shows up with a cure to the parasite that's slowly turning him and Ashley into mind controlled Ganados, just for him to get killed and the virus stolen. Then he has to fight through an island of enemies to save Ashley who was kidnapped again, at one point coming close to killing his love interest Ada due to said parasite. By the time Mike shows up to offer him some much needed assistance, just to die by way of rocket launcher, it's really surprising that Leon hasn't snapped.
4's ending credits...Damn. Seeing how the villagers had a normal life before Saddler started to use them for his own plans brought a tear to more than one gamer's eye.
Also how you see the villagers had children before Saddler arrived and injected them with the parasites, and there are no children to be seen in the actual game. Whether this is simply due to an error of communication between the game writers and the illustrators or it's actually implying something happened to the children before Leon got there is unknown, but there's something so chilling about it.
There are human bones in the fireplace of the first house you visit. Little human bones.