Abbys death in the third Chapter. Trying to keep up the barricade to save the others, she isn´t able to save herself. So in the end she is forced to shoot herself, to prevent becoming a zombie as well, leaving behind Alistair and her children.
In Chapter 5 Isabelles death: The first and final kiss with Ethan, the way she still maintains her confidence and finally her desparate attempt to reach the others by swimming after the boats, just to drown in the end, without ever meeting her mother... At least she goes out with a smile, thinking of Ethan.
Before the journey starts, Derreck and Eryn meet up one last time. He's being exiled, she will soon start the adventure. She gifts him her amulet. Derrecks answer is so much harsher in hindsight:
Derreck: That won't the last time we see each other. I promise.
Eryns argument towards Sara when persuading to give Sylvia a better job to care for the baby.
Eryn: Hey, soon Frank, you and me will sit in the bar together and whoever helped us caring for Richard will get a few free drinks.
Frank, Eryn, Sara and the child will all die.
Wills death is heartbreaking - he tries to protect the other people on the float and dies because of it.
After the events at the destroyed Shengs Hope and the Alamodome, Haile thinks that everything that happenend is her fault. She's not completely wrong, though.
Also, Shengs devastation after the events in San Antonio is pretty sad. He wanted to die, wanted to help for the first time in his life and he just couldn't do anything to keep his people (and his daughter) save.
Eryns slow progression into sadness and the spreading of her infection is absolutely terrifying and heartbreaking.
Where do we even start with station 7? The whole situation in itself is a tearjerker - after 20 years it's finally possible to find a cure for the zombie virus, but there is a whole army trying to stop our heroes from making it.
To start things of, Wingman, the paranoid soldier who organized the armory, sees a beautiful bush and vows to travel the world, if he lives. He survives the battle, so there's nothing holding him back now.
Franks death was pretty heartbreaking, too. He jumps in front of his wife, directly into the axe of an cultist and dies knowing that his family will survive the war. Extrapoints for the OOC-situation where the player got significantly better at writing as the plot progressed and became a vital part of the coregroup of players.
After the 'bombastic' death of Seeker Vulture there is only a small paragraph of text left - nothing to fear, right? But then, in a completely heartbreaking and utterly surprising manner we learn that You Are dies from her battle wounds. It's even more heartbreaking if you think about it: Romero did everything to keep her save and she dies looking at him and thanking her gods that he isn't watching.
The return of Derreck is really shocking - he's not human anymore and gave his life to keep the group (and Eryn) save on their travels.
Eryns death ist the mother of all tearjearking posts in all of the seasons. It's not only the music, it's the simple, little things. Eryn makes her last vow true, keeping Haile and Raoul save and in her final moments she reunites with the zombiefied Derreck. Both die, embracing and kissing each other.
Hailes complete moral destruction after the betrayal by Raoul. The girl without fear has given up.
The enemy is slain, the teenage lovers have survived and everything will be alright? Well, we can't have that, can we? Unbeknownst to the characters (and their players!) Raoul would have actually died in the showdown, but gets saved by Kerosas heroic sacrifice. Her death is heartbreaking and so sudden and without any warning, that it led to a lot of screaming and an really weird voicemail left on the Game Masters phone.
The almost-death of Sheng and Evi's desparation to save him count, too
Pray sacrificing himself to keep Blades and Jackman alive
The ending is another tearjearker - this time, it's a good one. Zooooommxbies ends with the names of all characters, major and minor, the ones who died and the ones who lived.
Another sad point to the ending is the fact, that there won't be a Shengs Hope ever again. Our heroes carry the cure in their bodies and have to travel the world to spread it.