I would say in response to this, 'people don't actually realize that not everyone is concerned with being a nice person.'

I knew an older Jewish women who walked with Dr. King. (She had an NAACP card that was older then I was) She was an awesome soul, and used to teach death row inmates collage level stuff.

She thought the same as you at first. That deep down everyone truly thinks they are a good nice person, that murderers and rapists all had something happen to them that 'turned' them bad. (Like a bad childhood etc.)

I'll tell you the same she told me. "I used to think down deep in each person was a kernel of some thing good, that it was just buried for some people. I no longer believe that. Some people are just evil, and are proud of their evil, and have nothing 'nice' or 'good' about them."

Some people act negatively not because there's some situational reason that turns them from being a nice person. Some people are sadistic, hate-filled evil and have no qualms about being a 'nice person'. Not because of some abusive drunk step-father, but just because they enjoy luring shy girls into their apartment to rape them.

I think this line of thought is more an expression of naiveté then anything else. Making statements this broad about all of humanity is usually a bad direction, even if it seems intuitive.