There are various fan theories floating around, mostly concerning the cause of Billy Joe Cobra's death. Feel free to include some of your own on this page.
- If Billy's mouth is to be considered purple for plot reasons and not stylistic reasons, it would suggest he died of metal poisoning. This is a very strong possibility as when Spencer was turned into a ghost in The Ghost of Spencer Wright , his mouth was not purple, and neither did Bobby in Copycat Cobra .
- Madame X is a crazy fan of Billy's, and in her stalker-ish efforts to get to him, she killed him so she could keep his ghost. Perhaps a stage light was unlatched during his concert, or she crashed into his limo. Either way, it would explain her fixation on collecting him.
- Billy is actually a government project, which explains his ghost-like abilities. He escaped, found the home of the recently-dead Billy Joe Cobra, and took on his name, personality and appearance. That's why he lies so often, and is so forgetful about Billy Joe Cobra's life, and doesn't think he's dead. (This has a lot of plotholes, considering all the references he has to his past life stuck within the series. Also, the ectoplasm?)
- A widely debated topic is whether Madame X and Billy have history from before he was deceased. A popular theory is that Madame X is a crazed ex-girlfriend or groupie. Light is shed on the fact that the creators may have named her "Madame X" as a pun because of this.
- A number of scientists have invited Spencer Wright to the machine where Danny Phantom became a ghost. Everyone in the room where the machine was located were turned into ghosts, and then they were turned back to normal. They tried again, only with Spencer in the room. Billy and Danny were involved in the incident (which crazy people think the Illuminati is behind).
- Billy may have overdosed on some sort of drug. This might explain why he acts so outrageously hectic and crazy. The fact that he is still affected by the drug in death would also explain why he doesn't have withdrawal symptoms.
- Spencer has superpowers, which Billy thinks is nonsense. Spencer has never got proof in when he saved the day from the movie accidents caused by Billy. Beverly Beverly High says 99% of Minions know he has superpowers, while only 7% of Beverly Beverly High students know about it.
- Billy wanted to make his fame immortal. In which he took his own life for the attention of the media.
- Billy trips or falls off stage or jumps off his roof to test a jet pack he bought or something else simple and silly. If the show were to mention how he died they might not make it heavy or serious.
- Billy was performing at a concert, things were going great but one fan really hated his singing so they plotted to hurt him when no one was looking the took a bottle of water and pored it on the amp, when Billy went to plug in his guitar he got badly electrocuted the shock ended up killing him instantly.
- Before he became famous, Billy was a complete and total dork. All Braces and excitement. However, he also had big self-esteem issues. All he wanted was to be liked and he often tried too hard. Even after he became famous, he still had some insecurities. He became narcissistic and arrogant, but not confident.
- Lolo has self-esteem issues. She just wants to feel loved and wanted, something she doesn't get from her father or her long-missing mother. She also wants someone who doesn't worship the ground she walks on and can say no to her. She often feels like a doll, perfect and hollow, when she really wants to feel like a person.
- Billy is based on the bass guitarist of the Sex Pistols, Sid Vicious. He had black hair, same hairstyle as Billy's, was arrogant and just like Billy, but he died in 1979 because of a heroin OD. He was one of the most famous people of his time, and he died when he was only 21. This can also be contradicted though because Billy Joe Cobra was a singer, not a bassist.
- Billy hugs/makes a lot of physical contact with Spencer because he never had anyone that actually liked him for who he was and not his fame, so he thinks of Spencer of a sort of life-companion.
- Billy Joe Cobra is Psychopathic. He often shows reckless behavior but a bit too reckless, and he doesn't really care for much people other than himself. In Scaring Mallory, he tried to run over Spencer and Mallory with a lawn-mower, in The Ghost of Spencer Wright, he tried to crush Spencer with a PA system, and in Evil Billy, he throws things at Spencer, tries to crush Dorine, Morine and Florine with lockers, and tries to drown/boil Spencer in a pot of hot cheese.
- Another theory is that Billy suffered from anorexia while he was alive, and this illness ended up killing him.