The mid-season finale of Riverdale revealed that Joseph Svenson, the school janitor, was the 'Black Hood'.
However, fans were not convinced, as the reveal was totally lack luster, and sort of didn't make much sense.
There was one creepy scene at the end that shows Betty burning all her Black Hood investigation in her fire, however when she goes to burn the hood, she keeps it instead.
Now, fans believe that she could be the killer.
So the theory suggests that Betty could have an undiagnosed dissociative identity disorder, pointing out some of the things that have happened to her.
The theory suggests that ~Dark Betty~ is taking the lead under the mask of the Black Hood, and since Betty never remembers what she does when she is acting as Dark Betty, it gives her a great excuse.