I loved the other books in this series/triology but this was a disappointment. 
 The Kill Order tells the story of Mark and Trina and the begging of the Flare. I expected it to shed more light on the Flare but there really wasn't much more information on it here. The only interesting new information we get here is why they infected people with the Flare; (view spoiler)[ after the sun flares that destroyed the planet too many people survived compared to the resources left. Unless more people died (about half the population of the US) mankind would soon die simply because there wouldn't be more food or other resources left. Therefor they gave "The Kill Order"; to unleash a virus that would eliminate about half the population. The virus was supposed to weaken over time and stop infecting new people. However, the virus mutated and become worse/more contagious that planned and they lost controll. (hide spoiler)] (if you don't plan on reading The Kill Order, but wants to know what more information we get on the Flare, this pretty much summaries it.) 
 That being said it wasn't completely bad. Dashner still has a lot of action in his book and he isn't a bad writer. The change of characters however made it hard for me to stay interested and I had to struggle to pick it up (I used almost a month to finish it!). The "love story" is simple and not really that good, and it seems like something he adds just because one has to have it nowadays. The similarities between Thomas & Tris and Mark & Trina are many and they could basically be the same person but with different backgrounds/memories.