Everyone knows what a vampire's greatest weakness is. Love. Something so simple, yet so complicated. All you need to do is figure out who they love, who they cherish... Then you have the key ingredient to destroying them. That was the reason why Klaus Mikaelson was so evil; he just wasn't able to find anyone to love him and he didn't know that he was capable of finding love in the first place. Sometimes, all people need is a little love and a little time, and you can change anyone.