I think that everyone, not just tarot readers, have had experiences with people with bad juju. There are people out in the world that... have few or no redeeming features as human beings. As to what causes them to be that way, I personally believe that I am not the person to judge why. Do I believe in demons/evil spirits? Yes. Do I have the knowledge to distinguish between someone being an a$$hole and someone being oppressed by a demon? No. That's totally above my pay grade.
Would I read for a person that fell into this category? As long as they were respectful to me, but I would take precautions. I would try to do the reading outside my home, if possible. I would schedule them towards the beginning of the day, when I'm a little more personally strong (for me, reading for strangers, like at fairs, is draining). I would use a separate deck for their reading, and basically quarantine that deck until it could be thoroughly cleansed (think deck inside a box, then that box inside another box with salt). I'd also take several minutes to cleanse/ground out any residual energy before doing a reading for another person, including myself.
So, as you probably guessed, I have done readings for people with extra-crappy energy. The best way for me to approach these types of clients is to remember that I'm just the messenger. I read what the cards tell me. If the cards focus on the negative, I do my best to convey that without judgement, and try not to panic them. I can (and have) recommended that a client talk to their priest/rabbi/clergyman or therapist.
I do believe in demons, psychic parasites, and the like. Again, as a messenger, I would let the cards drive where the reading goes. When someone seeks out a tarot reader, they have a fundamental question or conflict they are looking for insight on. This is where your personal instinct kicks in. If your gut says "don't go there", then don't.