There isn't a lot of fancy-pants symbolism going on in The Kill Order. This book is mostly meant to provide a) awesome fight sequences, b) loads o' death, and c) background information—like, a lot of it. That's why it's a prequel. But one thing tends to stand out, and that's the sheer amount of heat we see.
Sure, we know about the sun-flares. We know that they burned people alive. But it seems like heat simply dominates the world after the sun-flares. Even in Mark and Alec's journey, things get hot, really fast.
When they're both running around the forest (which by the way, is on fire), Mark feels the heat—literally, and figuratively:
Intense heat pulsed across his body as he ran; sweltering wind from the left, cooler air from the right. His clothes were so hot against his skin, they felt as if they might combust at any second even though they were drenched in sweat. (27.13)
This feeling of combustion and heat exhaustion overcomes Mark countless times—the heat makes everything seem worse. It makes Mark feel uncomfortable, sure, but it also disturbs his nerves:
It felt as if the heat inside the Berg had skyrocketed to a thousand degrees. The sweltering air combined with his shot nerves—the razor's edge of madness that cut into his psyche—put him on the brink of losing it. (64.9)
Ever been in an overheated room in the middle of winter, while you're wearing a sweater? That sensation is the closest we've come to being inside the fires of Hell. Well, imagine that prickly, sweaty sensation… but now imagine that you're not only hot, but your brain is being broken down by a virus. That's how Mark feels.
But what does all this heat symbolize? Well, it's actually pretty simple. If you've heard the phrase "if you can't handle the heat, get out of the kitchen," you'll know what we're talking about: heat in The Kill Order is synonymous with intense, overwhelming pressure. Whenever there's heat, the situation is about to get heated.
No wonder the cover looks so fiery, right?