How can a person be so sweet as a child, and end up a malicious human being who is only kind when their being fake or need to be?

What if they were never sweet? As a child you pretty much get every need taken care of by adults and your needs are pretty basic so you're never truly lacking so even if you are an evil manipulative twat you don't need to act it because you already get it all your way because you are small and cute. However as you grow your needs become more complex, the people around you stop giving in to all your whims and your antics instead of "awwww" get you in trouble and yelled at/punished etc so you reveal your true nature when you show how you deal with these obstacles between you and your goals. I don't think people are less evil as children, they just have less reasons and chances to practice their evil skills then.