Nikolai was displeased by her choice. He wanted this damned archer to suffer for fooling him and luring Sergio into the trap of Helnes Family. But, in the beginning, he did not hate her even though she attacked him with a holy arrow.
However, it was different when he saw her with Valkyrie. The reason behind all of his suffering was the careless whims of that woman, who thought she was a god that could not be struck down.
Nikolai wanted to leave Sergio's life forever because he knew that he would keep hurting the alchemist, endangering him even more than in his past life. It was the decision he carefully chose after admitting that he cared about his friend even if they had nothing to do with each other.
However, this lying, deceiving, and scheming mute wanted Valkyrie to use the alchemist. This was not a safe path for him because Valkyrie would discard whoever she could not use. Furthermore, Nikolai knew of her final goal.