I still think you are being narrow in the definition of fun. No, it is not fun in the sense of "I feel happy that I am killing this innocent creature". It is fun in the sense of "This game is engaging me (emotionally, visually, mechanically, etc) and I have the desire to continue playing it". Maybe 'fun' isn't the perfect word for it, but that is what people are implying when they talk about a game being fun. In the examples you give, the player is engaged by the game and wants to continue playing it, even if (s)he isn't filled with a warm and fuzzy feeling when doing so. And in the context of this topic, boss battles that make the game fun are going to further engage the player, be it through providing gameplay pacing via a difficulty climax, or by acting as pacing mechanisms for the plot, or in some other manner.