A lot of people need a villain in their life. They need a bad guy; someone to carry the shadow or the blame. Someone they can be above or feel better than. This is an awful handicap over time because it prevents a person from looking at their own character.
Who does this?
We all do this to an extent, but astrologically speaking, people with oppositions between personal and malefic planets are more prone. For example, I grew up with an Aquarius Sun opposite Pluto. She was light and airy, humanitarian, intellectual (Aquarius) and I was the scum (Pluto).
It very easy to spot something like this in a chart and/or in life, as these situations tend to be highly defined by their nature.
Why do people do this?
I don’t think people choose to do this, consciously. When you have an opposition in your chart, it’s impossible to simultaneously act both ends of it so we get others to fill in. If you don’t have an opposition in your chart, it’s the 7th house they gets projected and again, it is not necessarily deliberate. In fact, it probably is not done purposely. It’s just impossible to be a light as air, Aquarian, and simultaneously be the gunk that is clogging a sink under the house(Pluto), so this is what occurs.
What can be done to prevent this?
Nothing can be done, unless you become conscious of your tendency. Astrology is helpful here because it provided and x-ray. You see what’s going on, right there on paper. Once you realize you have this proclivity, you can work to integrate the parts of yourself that you put on others. For example, in the scenario with the Aquarian, she might really that projecting the shadow onto this person outside herself, but in her environment, is a very dark thing to do.
How can a person solve this all together?
Check your chart for oppositions, particularly oppositions between personal planets and Mars, Saturn and the outer planets. If you have an opposition like this in your chart, consider it suspect. Same thing if you have any of these planets conjunct the descendant or in the 7th house. Try to see how these less pleasant energies exist inside of you. Work to integrate the energy and find a healthy way to express it.
Psychology teaches that if you are meeting something over and over and have a strong reaction to it, you probably have the qualities yourself. This is the same principle.
What energy do you most often project? What is the astrology?