Defense against the dark arts pattern
the professors who circulate through the DATDA position all follow a pattern: first there’s a traitor, then the next one is an antagonist who is out for their own personal gain and doesn’t give a crap about the school, and then we have someone who is first thought to be a villain but later revealed to be an ally:
Year 1: Quirrel - Traitor
Year 2: Lockhart - Cared more about his fame than the welfare of the students
Year 3: Lupin - At one point we think he’s a traitor but he’s actually not
Year 4: Moody/Barty Crouch Jr - Traitor
Year 5: Umbridge - Cared more about the reputation of the ministry than the welfare of the students
Year 6: Snape - During this year we think he’s a traitor but we later find he’s the hero of the saga