What I have found over the years is that most kids are capable of mean behaviour, even the ones whose parents come on here and complain about how mean the other kids are...
I've witnessed many a time kids who you wouldn't typically think of being "mean" saying unkind things, or laughing along with other kids at other people's expense. Even kids like the OP's I"m sure will have times where they haven't always been as kind.
But every parent seems to think it's always other people's kids who are the problem, never their own, who are always "so sweet and kind". I'm not suggesting it's always to the same degree, as some kids are definitely more mean than others, I'm just trying to point out that ALL kids have their moments.
My DD is generally regarded as a kind person, she's head girl at her school, has a wide circle of friends and her friends' parents love her... But she's not perfect, and I have seen her be unkind a few times. We talk about it and she recognises her behaviour, but she's just a kid who, like all the others, are trying to find their way.
Some are definitely bullies, some get kind of dragged along with the bullies, some are insecure and do or say things they typically wouldn't because they don't know how to assert themselves properly or just say things in the spur of the moment, and some will be the ones who are picked on, but for the most part, most kids will fall somewhere in between bullies and bullied.
I'm sure I'll get people telling me I must have been a mean girl, or I'm bringing my child up to be horrible or whatever, but I'm just trying to point out that ALL kids can be unkind at times, and as parents, it's worth recognising that, rather than thinking our own kids can do no wrong and are always the ones being picked on.