The main reason why I personally think Bradford can share the same main antagonist role with Magica and General Lunaris because even though he is the main cause of the events, he still plays a huge role, being in fact the cause of the actions of the main antagonists (Magica and Lunaris) and if not for him, most of the events of seasons 1 and 2 would not have happened it’s just he makes little physical appearances and does little in the first two seasons, every villain had their own form of relevancy throughout the seasons, simple as that and there doesn’t have a sole Big Bad for a series like this because every season centered around a different one, there was never necessarily one particular Big Bad compared to many other shows, so it’s not rocket science to say there was a simply a number of different sole high role villains.