The entire outbreak was caused by a lovesick recently married couple from Las Vegas. As cringe as the circumstances might have been, it's upsetting to see two people's lives end before their life together could even begin.
The opening montage starts with horrific and hilarious sequences, but as the song becomes more melancholy, so too does the content:
Soccer Mom and Vanderohe attempt to rescue a couple trapped inside their car... only to find them both with bite-marks. Soccer Mom can only grimace as she mercy kills both, despite their pleas. Afterwards, she is seen drinking heavily.
The hapless paratrooper getting Devoured by the Horde as a result of poor planning. You can see the panic in his eyes as he wildly fires his gun, to no avail. And he was just one of many paratroopers who died that way.
While Soccer Mom is going through the pile of clothing that is being set for disposal, you can see minorities being caged and hosed down, while others are summarily executed, to prevent the spread of infection. But based on the treatment of the camps, there's no guarantee they were all bitten.
Finally the death of the Soccer Mom herself along with her daughter at the start is this especially with just how close they got to escaping. For added context, the Las Vengeance Crew is getting the last of the survivors out before the city is walled off with containers. Soccer Mom and her daughter are almost out when the former gets dragged back by the zombies. But her daughter cannot bear being separated from her again and runs back for her. Scott is forcibly held back by soldiers, while the while the final crate is dropped in place. The mother and daughter share a final embrace... as the crate crushes them both. Scott and Cruz are left mournfully staring at the now completed barricade, holding tens of thousands of zombies, so many of them people that they failed to save.
Just how miserable both Scott Ward and Maria Cruz are following their survival of Vegas events, despite the fact that they are proclaimed as heroes.
The flashback of Scott Ward being forced to stabhis own zombified wife in the head - and Kate herself first having her mother rabidly try to kill her, then having to see her mother's corpse.
The death of Chambers, especially with just how far she got in trying to survive - only to get overwhelmed, within mere feet of freedom.
The estrangement between Kate and her father, especially as they only reconcile as he is dying, with Kate being forced to tearfully do the right thing and shoot her infected dad herself.
The reaction of Zeus to the death of his Queen and baby, played entirely somberly by the movie.
Maria Cruz confessing her feelings to Scott, only to get very suddenly and gruesomely murdered shortly after.
The Heroic Sacrifice of Dieter to save Vanderohe. Made all the more tragic by the later revelation that Vanderohe was already unknowingly infected.
Bar one, the movie has a major Everybody DiesDowner Ending with the implication that another outbreak might be about to start.
The real tragedy in all of this is that no part of the plot had to happen. The zombies were successfully isolated from the rest of civilisation and were content to keep to themselves, the nuking was little more than the president swinging his dick for the whole world to see, the heist was a farce to get some alpha zombie blood and Vanderhoe ends up becoming the new Patient Zero in a country that most definitely can't contain a zombie-outbreak like the US could. Most zombie flicks show that the survivors are forced into survival and can be forgiven for acting unscrupulously at times if it means beating the horde, but this movie shows what happens when the horde has already been beaten.
