just check out this paragraph.Klaus is talking about Katherine:
She was always drawing unflattering comparisons between us. I tried to beat some sense into her, but it never really worked. Maybe I should have just killed her myself, I don't know. But by then I'd gotten used to having her around. She never was the brightest. But she was good to look at, and she knew how to have fun. I showed her that, how to enjoy the killing. Eventually her brain turned a little, but so what? It wasn't her brains I was keeping her for."