He managed to surpass Dazai by faking his death and escaping and getting the book finally all alone by himself, he made it to Russia, and no one knew of it.

It was victory for him at last, no?

All he had to do was write all he needs in the book!

But here you were, his greatest enemy, in the middle of his destroyed house, ready to bring him one more mission to finish before getting to his final victory.

“Was destroying the place really all that necessary?”

You can see his hand is gone.