To support the statement made by the title by citing my mother's and my experiences with two individuals, namely my grandmother and a man I only know as Jamie.
First, Jamie. While my mom still lived with me, he would visit with moderate frequency. What I say then was a gentle, compassionate, pleasent character. However, once my mom moved in with her boyfriend and Jamie, the dreadful truth was revealed. At home, he was a mannerless, selfish, apathetic, brute. He was also seemingly misogynistic, ordering my mom around to do various things. He also had some curious personality quirks. For example, he hated plants and anything plant-like. He also didn't like cleaning since he believed that the dirt made the place look lived-in while the quantity of the dirt, to him, was a record of his stay. He was also very overweight, with eating habits to match. I once saw him in front of the TV, sitting in an almost lewd position, eating nothing but five sausages, one-by-one, with his fingers. Ugh. He also didn't have much of an emotional range, leading me to speculate that he may be borderline-psycopathic.
The other person is my grandmother. Initially, she was a fairly pleaseant woman, albeit overly exuberant when expressing her love. However, after a while, her flaws became apparent. First off, she was obsessed with order and perfection to the point that she was incapable of percieving either. She demanded that everyone follow her rather lofty standards while barely bothering to achive them herself. She was also paranoid, partly a product her large ego, failing memory, and arrogance. She accused my mom and her boyfriend of taking her stuff, tampering with records, renegeing their part of the mortgage, yadda, yadda, yadda. The person who bore a very large part of the assault was my mom's boyfriend's son, whom she accused of taking marijuana, among other things. She also accused my mom's boyfriend of signing a contract in disappearing ink and minupulating my mom (I used to think it was the inverse). She also prized objects a bit too much--during her moving out, I remember getting disgusted at the look on her face as she looked at one of her prized dolls--it was a spectacular materialistic pride. She also took a chainsaw that she paid a part of, insisting that she be paid back for it. The early morning before she left, she had a fit, accusing my mom of tampering with her records, which I know isn't her style. So, the next morning, I exploded on my grandmother. Now, she hates my guts, but in retrospect I could have said it to her better.
There you are, two examples of people with dark sides.