This wiki page claims that Bill has killed titans, but I don't think that's true-- or at least, it can't be decided. In "Edge of the World", Bill says something along the lines of: "then we'll finally go from titan trappers to titan slayers!" Implying that he has never slayed a titan, only trapped one, or so he says. Unless someone can provide solid proof that Bill has in fact killed titans, I feel any text mentioning his killing of titans should be removed, tweaked, or include an added disclaimer that it's not fully determined...
Villains Wiki