(someone is) a spoiled brat
A "brat" is a child who is selfish and badly behaved. When someone is "spoiled", it means that they get anything they want. So a "spoiled brat" is a kid who has a bad personality because they get whatever they want.
You can call someone a "spoiled brat" when they selfishly demand to get what they want and don't think about what other people want.