Did i miss something??(spoilers)

No, after the klutzy way he was, at last, informed by the lazy, self centered weak boy who tried to shoot him, he obviously was not in a good humour and tried to scare the little twerp.
In fact this film is really about nothing and is as boring as the landscape which the cameraman films endlessly thinking that is "art".
Some of the suspense is well done but the rest is rubbish.
The script and storyline are full of holes so that anything the director was intending to mean or say doesn't happen or just gets lost.

I agree that Sergiey was trying to scare Pavel by the boat but I think the shot in the house was because even though he knew Pavel was there that perhaps he was jumpy because Pavel would not answer and Sergiey didn't know how much he scared Pavel and just assumed a polar bear was present.

Sergey never tried to 'hunt Pavel down'. When he first heard of the news about his family, he was in shock, so took a few steps towards Pavel, which Pavel mistook as an act of attack.

Pavel fired a shot at Sergey, which angered Sergey who in turn tried to scare him by firing a couple of shots towards Pavel. The first shot was intentionally close to him, and the second was into the air, to make Pavel realize how dangerous it could be. Sergey never expected Pavel would run away and really expect he was trying to kill him.

When Pavel does not return all night, Sergey goes out to look for him. When he is in the cabin on the cliff, he calls out for Pavel a few times, to which Pavel never responds. Instead he keeps quiet making creepy (glass breaking) noises. Sergey is cautious - it might be a bear - given the history of the cabin, especially since there is no response to his calls to Pavel. The sudden noise startles Sergey, and makes him shoot, which Pavel takes as a confirmation of his fear that Sergey is hunting him down.

The whole thing was a big misunderstanding.