Tom Marvolo Riddle started both Wizarding Wars, created the Death Eaters and killed Harry Potter’s parents.
Dolores Umbridge was spiteful, prissy and said to resemble a ‘large, pale toad’. She may have behaved like a monster but, technically speaking, she was a human being.
Highly intelligent and fiercely idealistic, his considerable gifts were nonetheless warped by malice. His defeat in 1945 by Albus Dumbledore was legendary.
Sadistic, tempestuous and fiercely loyal to He-Who-Must-Not-Be-Named, Bellatrix thought nothing of murdering her own family when they stood in her way.
Proof, perhaps, that mediocrity mixed with cowardice could be as dangerous as outright evil, Pettigrew defected from the Order of the Phoenix and betrayed those closest to him.
A savage werewolf whose speciality was biting children (most notably a young Remus Lupin), Fenrir Greyback was said to consume human flesh even when there wasn’t a full moon. Ick.
Sent to Azkaban by his own father, Barty Crouch Jr let his mother take his place and led Cedric Diggory to his death at the hands of Voldemort.
The equally awful sister of Amycus, Alecto somehow gained a position teaching Muggle Studies at Hogwarts.