This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Dead Island
The trailer. Holy crap. It opens with a shot of a deadlittle girl falling out a window, and the rest of the trailer depicts her parents' doomed fight to save her. And then the trailer ends with clips of the family's vacation video from before the zombies killed them.
Actually, it's even worse, she's not dead, she's zombified. She was caught trying to get into the hotel room, brought inside after a heroic effort from her father, and reanimated while her parents were desperately holding off the zombies, at which point she bit her father's major arteries out and fell out the window
The trailer is also shows, once again, that putting peaceful music to a scene of utter horror/violence makes it incredibly sad
Case in point: Youtube has plenty of videos with the title, "Dead Island Theme Makes Everything Sad".
It becomes even sadder when you start playing the game. You have to break into a room to make it out of the hallway. It's THE family's room. The mother and father from the trailer are there! What makes it REALLY sad? IT'S THEIR BODIES, STILL HOLDING HANDS.
Not even Tobuscus who does his "Literal Trailers" could make this funny. In fact, just the opposite. Dear god, the trailer was bad enough, but having it bluntly described to you as you watched it? Dear god.
"I'm not crying, there's just something in both my eyes." It's the broken, shaky way he sings that lyric is what makes it so heartbreaking and depressing. Especially 'I'm not crying'. He seriously sounds like he's on the verge of breaking down and crying.
The E3 trailer includes a moment where a weeping woman aims a gun at a zombie and says "I love you Dad" then fades to black when we hear a shot ring out. The game provides the context for this. Jin's father was bit before you arrived to get him to modify the armored car, and he locks himself in the garage and orders Jin to leave with the survivors. Toward the end of the game, she returns to the garage and shoots him.
This game was one of the only ones to make one feel sorry for the Zombies...or rather, what they used to be. Sometimes, when you hit one of the Zombies, it lets out a shout of pain. A completely human shout of pain. At first, one thinks nothing of it. Then it hits you...the human screams are supposed to remind you that these Zombies were once human like you. People who just came to the island to have a good time, or to get away from it all. And this shit happens to them.
The Suiciders actually ask for help in a very quiet, very somber voice. Then they explode.
Fairly early on in the game, there's a man who asks you to take his wife to safety, as he's been bitten and knows he doesn't have much time left. You can also find a survivor who will ask you to find his family at the church and bring them his will, which is written in blood.
There's also the guy near the beginning of the game who is sitting in a swimming pool filled with blood, and littered with the corpses of his family, who he had to kill.
What makes these NPCs especially sad is that you cannot interact with them at all. They just sit there going crazy, and you can't even escort them to safety.
What is even worse is that oftentimes, when you go back to the spot where you found them crying over a family member they just killed, a door is bashed in and there are bloody streak marks that show the person being carried away.
You cannot change or avoid much of this like you would be able to in some other games. The part at the beginning is right where you have to go to continue the game, and your character reacts the same horrified way they do upon first hearing the Infected.
Roger Howard's eleventh tape recording, in which he details his rapidly depleting physical and mental health as a result of the virus taking effect. All whilst desperately clinging to memories of his wife and son. It becomes much worse when you find his final recording right next to his lifeless corpse.
Jin's mental breakdown. She starts off as a very hopeful and compassionate girl, eager to help those around her. The she gets gang raped by the guys at the police station. Soon after that, her entire personality changes. She stops caring, becomes visibly distressed, starts nervously rocking back and forth, and completely loses it when she's forced to remain with a group of inmates at the prison. Then she dies. Poor girl had it rough.
Yerema's father sold her into slavery, where she was repeatedly beaten and raped. She ran away, and upon finally returning home, her dad sealed her away in a cave, before trying to murder her.
Dead Island Riptide
Riptide follows tradition with its own sad debut trailer. At least no zombie children are involved.
At the end, the survivors who stayed at their location on the island to wait to get rescued were last seen getting overrun by the zombies.
The player characters get it even worse. Regardless of what happens they are well past fucked. If the infection does not take over then they have become a lot more aggressive and their attitudes put the other survivors off side, even if they do not become zombies (quite possibly Super Infected) their infection has begun to cause an Unstoppable Rage, that cannot be controlled. Everything they have seen and done would be something they would have to live with for the rest of their lives, which would never again be normal, at the very least they would be closely monitored, or locked up, experimented on, or even killed because of the danger they pose. At this point death would be a mercy.