I think the same...I had a bad experience a year ago and people have told me it must have been a demon trying to possess me. I had the awful feeling of something dark and violent trying to push its way inside me and I was seeing awful images of...well nightmares. I kept hearing voices and things you'd hear in a thriller movie about the devil. It was quite horrific.
Now things are better and I don't feel so much of that but yes, I think demons can be quite evil.
I also think they can rebel against their leader in hell and try to be good but usually struggle.
Dears, I would not consider demons nearly as bad as either of you think. Demons are the same as any other spirit is; they are as able to hurt you as they are to help you, and they are not afraid to show force when they want something. For the most part, spirits will not be interested in a person, demons included. There are, however, those that will look to cause you trouble or strife, simply out of being bored or whatever else. They are as able to be nasty as anything else out there, and many of these are "lower" level spirits that will try to make your life less comfortable. These spirits can also easily become angered by you, such as the fae, land wights, and other beings of the earth. For the most part, these beings will cause you misfortunte, because you have directly done something against them or the places they inhabit.
However, unless you are particularly sensitive, you will not be able to tell the difference between a spirit involved problem and a non-spirit involved problem. For most of those that are not sensitive, not only will you not notice them, but they will not notice you. This is one of the many reasons why some people can sense spirits, but others cannot. I also think the "demon" Jinx experienced was a form of spirit or possibly a self-created demon. These self-created demons are demons of our own making, and not spirits in and of themselves. They can possess us, control us, rule us, etc, but only if we allow them to, as they are something similar to thoughtforms, but are not always sentient or existing outside of our minds. I think you should look deeply into your experience, and reject any opinions you and others have formed about the experience up to now.
I primarily agree with Mesu and PreacherCort. In Eastern religions, Abrahamism, and Christianity, demons are deemed impure spirits. However, Demons are known as Daemons in ancient Greek religion and Mythology and could be either good or evil.