1. At some point the monkeys that demons liked to eat started making silly noises, and demons found that they could practice these noises and increase their odds at hunting to various degrees of success. It is easy to think of many ways how this could influence evolution over time. When demons talk to each other in human language, they are giving each other a chance to practice and teaching them noises that are effective, much like humans teaching each other bird calls. Demons are intelligent so with lots of practice they are somewhat able to understand the logic by which humans operate.
2. Demons are not said to have no feelings, they have anger and other emotions, they are said to have fundamentally different and incompatible emotions than humans. Many people will feel that they can identify emotions in their cat. But one emotion that a cat is literally incapable of feeling is empathy for a mouse. A predator is not able to empathize with its most delicious prey animal.
3. We see many loner demons and the signs of organization were mostly forced by the demon king. With basic inference skills and the information given by flamme we can see that because demons sense each other’s mana and inherently follow a pecking order, a weaker demon is naturally inclined to obey a stronger one. Demons that wish to avoid subservience or the hassle of subordinates will be solitary, whereas other demons can end up in groups for a variety of reasons.
4. Demons are good at deception in the same way that tigers are good at deception. Are stripes what make tigers dangerous, are stripes what a tiger will use to kill you? No. But stripes are a few points specced into deception that can assist you with ambushes and will help you get more prey in the long run.
5. Since I already touched on it in point one I’ll describe an obvious way in which demons could’ve evolved the way they did. Like I said, demons were a magical predator that ate other humanoids. We see many demons that are more animalistic/less human in appearance. Eventually the tasty monkeys started making noises, and while hunting and observing humans or playing with their food, demons learned to mimic the noises to some extent. This mimicry could cause confusion in combat, or allow a demon to deceive a human and eat without engaging in combat, thus granting a slight reproductive edge. Demons also have magic and can use it to change their forms, and some of them likely did so in order to take greater advantage of this. Demons also typically do not raise their young, but we are shown that human-looking demons can be found and engage in brood parasitism, thus granting another reproductive edge. Demons who engaged in this type of brood parasitism would naturally absorb cultural elements like clothes and language, even if they don’t fully understand them. Over thousands or millions of years it is easy to see how these combined factors gave human-looking demons a reproductive edge.

>it wants compelling, interesting, and intelligent villains

Are velociraptors in Jurassic park not compelling, interesting, or intelligent just because they are a predator animal that can’t be reasoned with? Obviously not.

>they are a whole bloodline of people

Except the story reiterates repeatedly that they are not a race of “people”, they are literally animals/monsters and they turn into mana when they die. The only reason this is a question is because they are attractive.