There is a popular theory, sometimes called the Batman Villain fan theory or Bruce Wayne crazy theory, which suggests the caped hero is a patient at Arkham Asylum. The theory says that Bruce Wayne developed a life-altering mental illness after witnessing his parents' death, and had to enter Arkham Asylum since Alfred wasn't able to properly care for him. Instead of training to fight the bad guys who hurt his parents, young Wayne's psychosis became more severe, causing him to imagine becoming the hero Batman. While this isn't how comics typically play out, it does sound more plausible in the real world.
The theory further suggests that each of Bruce's imagined villains are a reflection of his own psyche, which he casts upon his fellow patients and health care workers in Arkham. Each villain represents a separate part of his mental state, as he uses his Batman fantasy as a form of personalized therapy. While some villains are insane, others are violent, and a few are sometimes caring. If you examine the main villains, this theory sounds more plausible.