Just off the top of my head, some of Terl's low points include:
- Nearly killing his captive by forgetting that the native lifeforms respire air, not breathe-gas
- Neglecting and injuring the creature his big scheme hinges upon
- Never bothering to learn his slaves' language, thus allowing them to plot against him even while he's present
- Accepting that two creatures have psychic powers, even though said powers didn't help them avoid the traps that captured them
- Taking hostages, then nearly letting them die through neglect
- Setting his automated surveillance for regular inspections rather than constant surveillance or random inspections
- Painfully unsubtle hints that he plans on double-crossing his "workers"
- Taunting a corpse with self-incriminating statements
- An inexplicable obsession with the idea of smuggling gold through fake coffin lids
And so on. Point is, Terl is hard to take seriously as a diabolical mastermind, and both his greed and stupidity keep allowing the protagonists to succeed.
Of course, to a large extent this is intentional. As early as Part One, Chapter One, Char tells him "you got your appointment because you are clever. That's right, clever. Not intelligent. Clever." This is also where Char expresses amazement that anyone would want to go out and hunt humans, directly contradicting the whole "evil bloodthirsty Psychlos hunting humans for sport" angle introduced later, but never mind. The point is, Terl is never meant to be as cunning as he thinks he is, and his ego surpassing his ability is his flaw.
Which is all well and good, I suppose, since characters need flaws to keep them believable (see: Jonnie), and there's the whole "evil sowing the seeds of its own destruction" thing. But Terl takes it to extremes, and most of his accomplishments come despite his best efforts (or completely out of nowhere, like how he cracks Numph's code). And like so many other things in Battlefield Earth, it's Terl's stupidity that allows the heroes to succeed more than the protagonists' own acts would allow. Terl undermines the heroes' accomplishments by inadvertently aiding them and devalues them because an idiot like him manages to be an obstacle to their success.
He's not a very good bad guy, when it comes down to it. Terl's a second or third tier antagonist like Jabba the Hutt or one of the named orc captains from Lord of the Rings. And yet, he's the closest thing Battlefield Earth has to a main villain. Because who else would it be?
Normally a story has a Big Bad, the incarnation and personification of whatever forces are opposing the hero. LotR had Sauron, Star Wars had Darth Vader and later the Emperor, Super Mario Bros. has Bowser/Koopa, and so on. These guys cast their shadow across the entire plot, and their defeat is usually a satisfying part of the climax.
Terl does not loom over Battlefield Earth's story. He's a major player for the first quarter of it, but he's dead halfway through. And he isn't the personification of those evil Psychlos, because he (one of the catrists' chosen, entrusted with the secrets of teleportation) is trying to swindle the Psychlos too. He's on his own side.
So who is the main bad guy(s)? Brown Limper, the jealous, power-hungry cripple? He's a pathetic imitation of dictators past and easily out-maneuvered by the heroes; the closest he comes to relevance is a few chapters in the middle. One of The Gray Men? They're only clearly antagonists towards the story's end, and never villains. One of the Tolneps, Arsebogger or Snowl or that captain whose name I can't be bothered to look up? Probably not. The Psychlo emperor? Never named, much less appearing in the story. The catrists? Jonnie only learns about them loooong after blowing them all up.
We can't even really say the Psychlo Empire is the force Jonnie is struggling against, since it never appears in the story aside from background exposition and a brief chapter set on the capital world. It'd be more accurate to say that Jonnie's opposing any hostile aliens rather than any specific character.
That is, if you view Battlefield Earth as a single story. However, if you break it into episodes, things work a little better - Terl is quite clearly the baddie of the "Jonnie in Chains" and "Liberation of Earth" arcs, while Snowl is the villain of the "Conference of Kariba" episode. Which I guess I'll have to explore further when I examine the story structure of Battlefield Earth.
This has been rambling a bit, so I'll try to wrap things up with: Terl is an idiot and not a very good villain, but Battlefield Earth doesn't have a consistent nemesis for Jonnie, so he's the closest thing to it. Which is sad.