Today I was walking and thinking how to answer your question. I saw a really dirty stinking bum in the street and then it came to my mind how to answer.
I imagined to take that person in a place to take a bath, shave his hairs, take a course on good education and treat him of his psychiatric problems. If we could do that, then that bum would be like anyone of us.
That condition is external, depends on external conditions, it has nothing to do with what he really potentially is, i.e. a decent human being.
Similarly the demon is a very fallen and contaminated soul. He has thick layers of hate, lust, rage and greed. But below all that dirt there is a pure spirit soul, shiny as the sun. In some time, maybe in a few lives, who knows, he will have paid the reaction of his sins and become liberated, maybe a gopa, a gopi, a great acarya saving millions of fallen souls.
Yes, as long as those layers of dirt will be hiding and suppressing his glorious svarupa he can’t go back to Krishna. It is impossible.
But there are various ways by which that soul can begin his rehabilitation.
The most common is that even in the most intense sense gratification something in the heart of each and every living being there is something telling us that this is wrong.
That feeling is a starting point because Paramatma notices it. Srila Rupa Gosvami calls that moment as sraddha, the initial impetus for knowledge. That can take one year, one life, or hundred lives. Nobody can tell when, but the the path of return has begun.
The second part of your questions says:
If one causes problems to others, is his karma and dharma to do that, and then what fault does he has?
Your reasoning sounds like this: if I have a contagious disease, why shouldn’t I go around and mingle normally with people? It’s not my fault if I am sick.
But actually you are innocent if you don’t know your state of health. If you know and you still mingle, you are guilty.
So the fact that somebody has a demoniac nature do not deprives him of the result of his actions, because he always have a choice. Just like some devotees, sometimes they misbehave. They have all instruments to do not sin. Perfect knowledge, good association and so on, but they still misbehave. Why? Because free choice is always there and our own nature is partially an excuse. He sins simply because wants to sin.
Demons hurts people simply because they want to hurt people, and therefore they are guilty.
– Manonatha Dasa (ACBSP)
31 may 2019