Wow, I came to this late. Still, last post was a month or so ago, so I'll dive in.
I have found that limiting Demons and Angels to the Christian definitions is limiting in unhelpful ways. I tend to define Demons as beings which represent or exert a force or personal nature that is anathema (intentionally or otherwise) to humans or perhaps to the world in general (note that I don't mean the human world, but some aspect(s) of the world independent of us). Some humans find kinship with these beings, take them on as teachers or allies. This just means that these people have a nature more in line with the Demon in question or some current of demonic energy in general. It also means most will likely not have this same experience.
Same with Angels. In my definition they are beings that exist to support either human existence or some function specific to humans, or they exist to direct/maintain/embody some form/expression of natural/universal order. Again, some people will jive with the energy of an Angel, some will jive with the energetic current of a group/tradition of Angels. Some will feel neither and leave them be.
Note that I am using immensely generalized modernized terms and that most spirits that fall under these ctegories will have a tradition they belong to and a traditional name for their class of being in that tradition, like animal/spirit guide to many native traditions might fit my Angels description, but I would not refer to them to their face or to native medicine practitioners as such. Similarly, some Jinn in middle eastern lore might fit the category of Demons as I have described, but I would not refer to them as Demons, I'd refer to them as Jinn in any work specific to them. I do find these terms as a great category, however, to categorize spirits in my own notes who do not fall into a predetermined cultural or religious paradigm.
Take from it what you will. I use these categories to help remove my own value judgements from entities I work with, and for that purpose, it suits me well. Let me know what you think.