Okay, this is gonna be a little weird, but one of my dad's cousins (a first cousin once removed????).
I always used to wonder why my dad and the rest of the older kids in his family seemed so damaged until I learned about this from my brother. He did some digging into the family history on account of my dad being the disturbed and melancholy soul that he is.
My dad's older cousin used to beat up my dad and break his stuff when he was a kid. My dad endured multiple humiliating beatings from him, and my grandpa looked the other way (I have issues with him too). If that wasn't enough reason to dislike this dipshit, he also molested my aunt when she was a little girl. Because it was the 50's and because my grandparents were naive and easily manipulated by my extended family, they didn't believe her, and to this day she is not believed by most of her relatives. This is despite the fact that this pedophilic SOB went on to molest another girl (now woman) in the family. His enabling parents (my great aunt/uncle) swept that one under the rug too even though the age gap was much wider and the evidence more damning for him.
My grandparents failed to protect their children, but it is difficult for me to hate them as their grandchild. They did eventually make the call to get the fuck out of town and move states - but only after my dad and his younger siblings had developed lasting psychological scars that affected the way they raised their own children.
That said, if forced to pick a family member that directly made my life hell as a kid, it would have to be my dad. Fatherhood is a hell of a ride when you hate yourself immensely and your wife gives birth to a kid who's just like you.