Dolores Umbridge, the worst villain in the Harry Potter series, wound condemned to Azkaban after Order of the Phoenix. According to Pottermore, the reason was that she was involved in imprisoning, torturing, and even ordering the deaths of several innocent witches and wizards during the Second Wizarding War. Being the worst fictional villain since Hannibal Lecter, on the other hand, is a fairly good justification to be imprisoned in Azkaban on its own. Was she worse than him who shall not be named? Maybe not, but wizards and witches like this one were crucial to his machinations, so she's surely in the running for the worst of the wizarding worst.