This may be difficult for followers of a monotheistic religion to accept: angels are not all goodness and light; demons are not all evil and darkness. Let me explain…
Angelic beings do not have Free Will – and the demon side are angelic beings. An angel will deliver a message of joy one day, and kill all the first born children of an entire culture the next, and feel neither personal joy nor remorse. They do what they do because they are told to do so by the Boss. The demons are fallen angels cast out when their archangel questioned whether giving mankind free will was a good idea (according to one story – there are others).
Lucifer, as with all Archangels, leads a legion of angels. Michael, Ariel, Gabriel, and the rest all have their own legions to carry out assigned tasks. All are structured like an army with junior and senior officers, noncoms and grunts. Hey, it’s a war out there. This structure is similar to a polytheistic structure, but the lack of free will in angels has no parallel in polytheism. Unlike polytheism, the rules are fairly straight forward.
For angels it is very simple: they can help and support you because, generally that is what they are told to do. They can bring understanding, protection and aid in “moral” decision making. What they can not do is prevent a fallen angel from getting its hooks in you because they can not interfere with your free will. Reread the opening statement for this section and be warned: an angel sent to teach you a lesson, whether it is nice or vicious, is intractable. They can not be reasoned, pleaded, or compromised with; you have to appeal to the source – remember, they are literally on a Mission From God. And they can be terrible and unrelenting in carrying out their purpose. However, if you call upon an angelic form for help, (and it comes) it will be equally unswerving in that purpose.
The demonic side has rules as well. First and foremost: the Devil (or any demon, really) can not make you do anything. They can be that nagging little voice telling you over and over how good things would be if…; they can increase doubt, push you toward excessive anger, or compulsion, and other destructive or confusing traits we all share; demons can show and teach you amazing things. But at the end of the day you have free will and the choices you make are your own. You are responsible, not the demon nor the imp (lesser demons).
Fallen ones do not lie. I know, Lucifer has been called the Prince of Lies, but look at the stories and everything he ever said to tempt someone was true. Partial truths, yes, they do that; what you believe true (even if it isn’t, if you believe it, you accept it as true) can be used; out right lies, however, are out of bounds. Everything the serpent told Eve was true: Eating the fruit of The Tree would indeed give knowledge and make mankind as gods above all other creatures; what he didn’t tell her was the price we would pay – eternal strife, disease and hardship; nor did he mention the burdens of guilt, jealousy, greed, and the rest of the deadly sins. But he did tell her the truth. They have to answer your questions truthfully, but use the same rules: partial and confusing truths are allowed.
If you are monotheistic, you already know how to deal with the Fallen. Call an Archangel (or the Boss), fighting off demons is a big part of what angels do. Or question it to death. How many times can a child ask, “Why? Why? Why?” before you overload? Works with imps; it just takes longer.