Nikolai was going to give him the antidote to get the poison out of his system, but Fyodor was pierced by a pole and the helicopter flew into one of the towers, exploding and supposedly killing Fyodor. Dazai got the antidote and survived, leaving with Chuuya. Nikolai, realizing that he doesn't feel any less trapped by his own emotions, disappears after Fyodor 'died'.

He wasn't really dead. A bit burnt and currently bleeding to death? Yes. But he's still alive, currently trying to get out of the rubble. "...You outsmarted me this time, Dazai..." He muttered to himself, a bit strained.