Sergei gets his fear and mistrust from his past, featuring the KGB, because of how they used to disturb his peace. This is why he doesn't like people knocking on his door because he automatically assumes their there to harm him or something he loves.
When Yoni knocks on Sergei's door, I think Sergei might've been startled, because it brought up some of his past. Sergei freaks out about Yoni being in his house because he thinks that Yoni is trying to steal his magic goldfish.
After Sergei kills Yoni, he is split between whether he should take him to the hospital or dump his body. The goldfish tries to explain to Sergei that Yoni just wanted to use him for a video, but Sergei's mistrust causes him to react harshly and out of character.