Hello,
I have been doing research in these things for some time now and I know what can make your soul a demon. Demons were a living soul just like any of ours but these souls just went through. Not all demons did a bad deed and is suffering in hell. The soul which has went through depression, fury, agony, rage, and all sorts of things before dying or giving up become a demon. The demons can be the soul searching justice for its pain, taking others life to make its own happy that its not alone. But some demons are actually bad while living as well as after death. Some demons are formed due to powerful curse. The strength of the demons depends how much fury it had inside of it. Some are just bad spirits but some become demon. Demons are not opposite to god, Evil is the opposite. Demons are the opposite of angels which are the souls which takes care of us.