The ambiguity of heroism
Throughout the novel, readers are left to consider whether Hester is a hero or a villain. Quickly, readers discover that Hester is neither a hero nor a villain. Instead, she is someplace in between. Although she engages in terroristic activities, Hester is also a freedom fighter who is focused on ridding the world of tyranny. For instance, even though Hester uses the MEDUSA system to murder helpless, innocent citizens, she does so with a good reason: to kill Valentine and free the citizens of London and other cities.
Climate change
As a result of the so-called "Sixty Minute War," the entire Earth's climate radically changed from relative stability to profound instability. In fact, according to the novel, the North Sea's bed became "dried out" and monsters were created because of climate change. These monsters were incredibly powerful and couldn't be dealt with very easily. In the end, Mortal Engines shows how delicate the Earth is and just how important taking care of it is.
Violence
Mortal Engines, and subsequent novels in the series, is set in a world consumed with violence. After a very violent war that didn't even last an hour, the Earth was destroyed and shaped into something new by the violence. Nevertheless, the characters in the novel don't like violence. For instance, Tom "tried not to think about Pewsey and Gench, burning inside their burning gondola." Still, he (like everyone else in the world), thought of the violence "every time [t]he[y] closed [their] eyes." Violence has the potential to change everything and has the potential to traumatize people around the world. It's up to humans to ensure neither of those things happens.
