For the first film, the death of Michael Ironside's character, Lt. Rasczak. His agonized "you know what to do" just before Rico is forced to shoot him doesn't make it any easier.
Dizzy's death, even if you hated her and are a Rico/Carmen shipper, it's still horrible to watch.
Made even worse by the fact it was probably Rico's fault. Any infantryman is taught that you NEVER pull out shrapnel as you will make things much worse. You bandage around it and leave it for a surgeon. Diz might have lived if Rico hadn't mutilated her insides.
Zander's death, yes he was a tool, and basically the guy responsible for Rico and Carmen's temporary falling-out. But you wouldn't wish what happened to him on anybody.
Zander was NOT a tool. He was an upstanding officer and man. He made it clear he wanted Carmen, he didn't sneak around. He kept his word when he fought Johnny about disregarding rank when he could easily have had Johnny kicked out of MI or lashed again for striking an officer. He risked his own safety to give the Roughnecks cover fire to retreat from Planet P when he could have sat safely in the cockpit. He just had the misfortune to be in competition for the hero's love interest and so was seen as a bad guy.
It mostly applies to the first movie, but seeing what humanityhasbecome is hard to watch. And the ending isn't much better. They're reduced to using CHILD SOLDIERS in a last bid gambit.
Especially divisive, but seeing the bugs get torn apart in a great big inferno during one scene is especially hard when you consider the theory that none of this might be their faults. They were just minding their own business when suddenly some monkeys show up and give them the boot. Sure, they kill without remorse, but none of this would have happened if humanity didn't decide all bugs must die.
The MI wasn't using Child soldiers. Most new soldiers are 18-19. They just look ridiculously young compared to the stars who are in their mid to late 20's. The contrast is deliberate to show how much war has changed Johnny and Ace and to invoke "were we ever that young?"