Is it that they feel this way inherently (maybe) or that there are a large percentage of people that surround them, and/or stand to gain from them, that tolerate and allow that behavior?

My feeling is that people act the way they do because they can get away with it with little repercussions.

If you've ever dealt with a person with an anger problem you will see that many times when they deal with someone close to them they act one way (abusive) but with complete strangers they can be as sweet as pie because that person is the unknown and they aren't going to take a chance with a stranger who might react in a way to deny them what they want (of course yes there are those who act like total loose canons with everyone but most that I have observed tend to differentiate between degree of closeness).