The scariest person I have ever met was a guy from my neighborhood, "Snowdog." He was a long time associate of my "uncle"; in prison they belonged to the same 'group' and they were crime partners when not in prison. Snow had his own son stabbed in prison for leaving him behind after he'd been shot during a robbery.
What made him so scary was that he could sense out weakness- you could literally feel him searching you for a soft spot. And if he found one, he would victimize you- and it was like he had to do it, not even like he wanted to, if that makes sense. My uncle's description of him- an albino silver backed gorilla- was pretty spot on. He was killed some years back, maybe 9 or 10, during a bank robbery.