I don't think she's as bad as him either. She's presented as a sadist, of course, but we don't know by how much [not everyone's the same]. In reality, all she did was taunt and tease, and kick and fought Sora a couple of times, beshslap Naminé [and personally, I think that was the first time she laid a hand on the girl, and it was only at a focal point where everything was ruined and she was in a worse mood]. Most of this can be fit to even fights in physical, as I'm sure a pair of pointy chakrams are worse than a hand slap [and somehow, people turn out alright on that one >.>]. Otherwise, she grabbed/forced Repliku to change his memories. Compared to a real sadist, that's kind of tamed. That's more like bullying, than anything. That's how I kind of see it, anyway.
Sadist is a kind of a person who likes to cause pain and/or humiliation upon others.
So the taunt/tease sort of put Larxene in Sadism category~ meaning: she likes to humiliate them and enjoys to see them being hurt by it. It doesn't have to be physical to be classified as sadism, it can also be verbal.