The easiest way I have found for this sort of task is to treat people as a gestalt, a structured group whose whole is different from the simple sum of its parts (which is not a difficult reach). This points out that once you're inside their "mind," it isn't homogenous. You don't need to have a uniform perfusion of perversity tainting their mind; you can have regions.
From there, I find it effective to think of the mind in terms of an inside and an outside. On the outside, there is a need to respond quickly to constantly changing situations. Reflexes sit on the outside. As you move towards the center, you get a more and more complete situational image because you can take the time to process more of the information. However, you also get a more abstract image because you simply don't have the bandwidth to soak up every single sensory neuron's data and forward it (it'd drive you insane).
So we have a archetypal "mind:" a region of fast-reacting reflexes on the outside which don't have a full view of the world, but they truly understand their little corner of it. In the middle regions, you have slower-reacting thought processes which take the time to process the entire world with more and more abstraction, feeding that information to the exterior (the sense of "the hairs raising on the back of your neck" would be an example of interior abstract models telling the periphery "you better get extra sensitive, we think the world is really dangerous and we need super-jumpy reflexes").
With this model, we can describe a demon with a negative trait deep in the middle of the abstract region of the mind (which is why the negative trait is usually some abstract concept like "gluttony"). Left unchecked, this negative trait will simply spread, completely consuming them and destroying any shred of humanity. That's the boring case, so let's assume they're not unchecked. Let's let our demon literally do war with its negative trait.
This process would not be simple. If the negative trait gets too close to the "center" of the mind, it would take over the most abstract and most fundamental concepts behind the demon's self, so the demon has to keep the negative trait away from the center. However, if the negative trait manages to get close to the exterior, it gets in a position to physically command the body because it can effectively cast a shadow over those exterior sections, preventing them from benefiting from the wisdom emanating from the central regions. From there, it would be easy to manipulate those small fast-but-not-so-smart regions of the mind to do the negative trait's bidding.
Accordingly, the best the demon can do is to hold their negative trait in a sort of holding pattern between the extremes, trying not to let it control its body, but trying not to lose control of its self by letting the negative trait dive into its center. The negative trait would have to orbit in the middle, seeking ways to poke and prod either in or out.
In effect, the demon now has an internal demon for itself.
Every now and then, the demon would make a mistake, letting the negative trait dive too deep towards the self. The demon would have to rapidly pay attention to this, hardening itself and directing awareness away from the periphery. At this time, the negative trait could easily give up its quest for the center, take substantial control of the body, and wreak havoc before the demon's self can wrestle control back.
This should be enough of a model to answer your questions, so lets address them in order
I also want several polar groups representing different fears like
rejection, lack of control, being chased etc, but is that
contradictory to depth, would they have a mixture with more dominant
dispositions complimented by weaker flaws or a harmony of flaws?
I think any individual, demon or not, facing a great flaw, would develop an inherent natural set of lesser flaws to help it out. A real life example would be the poor father with no self control developing a drinking problem to try to drown his sorrows.
Edit #1: If you have characters founded on extremely negative traits
how do you humanize them?
Show the struggle. Self awareness of one's negative traits and the effort to overcome them shows humanity, even in the worst. The TV show Scandal had an excellent example of this in the form of a character that had gotten addicted to torturing people, and was basically putting himself through the worst 12 step program any human could ever deal with. In one horrible moment, where he had to return to torture, his words were something to the sense of "I don't want to torture you. I fear I would enjoy it too much, and I have spent too long try to get away from that black abyss. However, this is very important. You are going to have to learn a lesson. So, I will enjoy it, as best as I can. I am sorry"
Edit #2: Can characters have depth if their flaws revolve around a
single extreme negative trait, like jealousy or greed? Or does there
need to be a mixture?
As long as the negative trait is not in a boring holding pattern, there's plenty of room for depth as we watch the parries and ripostes in the demon's character as it tries to grapple with the negative trait for good. Consider that, in Chinese mythos, yin and yang are only two forces, and yet their interplay is support to bring forth all the excitement and color of the world.
Edit #3: Is it plausible for a community to be driven in unison by a
singular internal negative trait?
Yes. The model given was for an individual person, but you can make almost exactly the same model for that of a group, as long as they interact enough to form reciprocity relationships. A negative trait hitting that greater group could easily place itself in a position to "settle" into the individuals all in unison.
For example, imagine if there is a village that is always angry, how
do they coexist with each other? If that's possible, could they
coexist with other villages that are always envious, greedy or
How to coexist is an open ended question. However, if I may point to ideas, Bruce Banner is "always angry," and that is how he makes sure he can coexist. I'd also point out that many religions provide a path to trying to overcome vices, and they usually start by ensuring members of the religion can coexist. I'd point towards Buddhism, because it seems to be the most direct approach that works this way, but I would not discount any other religion's solution, because they all provide this support to their worshipers.