When people think negative thoughts, act selfishly and destructively, they attract negative entities and elementals into their energy fields. Such creatures dissipate their life force and tempt them to act in a yet more evil way.
When you feel very negative energy from someone, it’s very likely that you feel not the energy field but those dark entities surrounding the person. By living a spiritual life you can get so sensitive that you almost physically feel those entities (your body reacts against them) and you want to be nowhere close to such individuals.
Also, once through negative thinking or evil acts people attract those beings, it’s very hard to get them out of the energy field, so it’s best not to give them an opportunity to attach themselves to you in the first place.
A good sign to know they are attached is if you feel tired all the time or you look worn out, irritable, etc. To get them out you should stop feeding them with negativity, alcohol, anger, aggression, lust, overeating, and so forth, depending on what attracted them to you in the first place.