Board Thread:Questions and Answers/@comment-62.1.165.98-20181011205728/@comment-6697950-20181011210425

Nobody but Kayaba knew that SAO would become a death game. As such, the game was created just like any other game and had assets prepared in advance, including the bosses. However, just because there was an asset doesn't mean that Kayaba had any intention of using it. The 100th Floor Boss is an example of that - while it did exist in the game's files, Kayaba was planning to scrap it and take on the role of the boss himself. However, just because he was planning on making such a change doesn't mean that the original boss was just deleted. It still existed on the server, which is why Shigemura was able to make use of it.