Sergei dislikes strangers, and resists Yoni's intrusion into his home. Sergei displays general distrust towards all people, due to his upbringing alongside the intrusions and threat of the KGB.
Sergei, anxious about Yoni's presence in his home and around his dear friend, reacts violently towards Yoni when he imposes upon the goldfish. Driven by paranoia, Sergei kills Yoni with a burner.
Sergei ends up letting go of the goldfish to save Yoni, but he comments later that he would do anything not to be alone. His paranoia of letting go of the fish is demonstrated even after he releases it, demonstrates his attachment to the fish. Sergei's paranoia stemmed from loneliness and he attached his emotions to a temporary companion.