Why do people call others 'whore' and 'slut'?

I know some girls that are as of now, pretty shallow and insecure people and they call others ugly, fat, whore (they even say this to guys), and slut (things like that), and I was just wondering, why would they do that?

Do you think it’s because they are insecure and they really feel that way about themselves? Because the people that they’re calling those names are mostly quite nice, honest, and moralistic people (not whores and sluts).