I'm not sure specifically which 'bill kills mabel fanfic' you're referring to. It could be a fan - created fictional story within a particular fandom where a character named Bill kills a character named Mabel. However, without more context, it's hard to give a detailed account.
One possible theme could be betrayal. If Bill kills Mabel, it could be seen as a huge act of betrayal depending on their relationship in the original source material.
One common theme is adventure. Since Bill is often associated with strange and otherworldly things, many fanfics might involve Bill and Mabel going on wild adventures together.
Another theme might be the challenges of a new relationship. In a 'bill marries mabel fanfic', they would face various challenges, like dealing with each other's families or friends. And this could be a major theme as they navigate through these difficulties together. For example, if Bill has a different social circle than Mabel, how do they handle the integration? Also, cultural differences if applicable in their fictional setting could be explored.
One common theme could be the struggle between Mabel's normal self and the influence of Bill. It might show how she tries to break free from his possession.
One interesting 'Bill Cipher x Mabel' fanfic could be 'The Strange Bond'. It explores a unique relationship between them where Bill tries to understand Mabel's world in a rather warped way. It has some really creative scenarios.
I'm not sure specifically as it could be a very specific fan - created story. It might involve the character Bill in some sort of scenario where he kills Dipper. Maybe it's a dark take on the relationship between these two characters within the context of the original work's universe.
One popular Mabel x Bill fanfic might be 'The Mysterious Bond'. It explores a unique relationship between them where Mabel's positive energy starts to influence Bill in unexpected ways. Another could be 'Mabel's Unusual Adventure with Bill' which takes them on a journey through different dimensions.