Yes, but do they really start out like that, or is it a gradual descent? There's usually a catalyst that starts it in kids. Parents demanding success at all cost / rampant cheating around them (or just being surrounded by ruthless kids) / a very competitive rat race that turns out to be pretty dirty / the parents themselves are without conscience / etc.
I think children tend to be pretty cruel when given the chance, many of whom grow up to be decent people. While there are certainly bad people out there who were already rotten as children, I don't subscribe to the theory that people are born perfect and then somehow experience moral decay as they grow older.