We see the chief Thern, Matai Shang, manipulating the chief Zodangan, Sab Than, throughout the entire film.Villains don't need motivation or explanation, becuase evil is just evil and needs to be killed--what more do we need to know to enjoy an adorable CGI space-dog?
Shang was also considerate enough to explain himself to John Carter, at some length and while demonstrating his fancy shape-shifting abilities, in Zodanga.
The evil, planet-hopping, chaos-eating thing isn't from the original. It's from the film. That's the gist of what Shang tells Carter in the scene I mentioned above. Popcorn break?So far, the only answer I've gotten regarding the Therns is from Mercutio, which is that they "appear to be evil immortals that feed on chaos and destruction". Now, that's from a fan who read the books, and he's not too sure.
FYI... in the books the Therns are just a race of degenerate Martians who murder religious pilgrims and take their stuff, so they can lead lives of decadent evil comfort in a spacious mountain lair.If that's what constitutes "clearly spelled-out" in your book, then we've simply got a big difference in standards.
Yes, their motivation is "kill things and take their stuff". In the context, it's fine -- though I personally prefer the new film's take.