I think maybe it didn't kill all of them. Maybe some of them died naturally, due to the cold. The two Yuris, maybe Zina, Igor and Rustem.
But when the unknown force saw that the group is still alive and struggling back up the slope, then it might have went to the forest and killed Lyuda, Semyon and Thibo, and perhaps Kolevatov?
They tried to hide but were found, or alternatively, as they were the only ones with injuries suggesting something sinister, the unknown force hid their bodies in the stream / under snow. Perhaps even triggered an avalanche there.
Because of the flashlight found on the rocks, they would have left the tent at night, then in the morning someone climbed the cedar, saw that the tent might be safe, and they started their return to it..
This also adds up with the "observation slit" origin of the cuts on the tent.I personally don't want to think it was murder... but it's compelling.
But why would the unknown force change its mind in the morning and decide to kill them?