Not sure it was Igor, although the conflict would almost certainly be between him and Semyon and come at the moment of highest stress on the trek, up on 1079, and in close confinement inside the tent, which wasn't ridge supported so the sides would tend to bring people into each other's faces more as they sat down to eat and rest a bit.
Igor's petulance on previous hikes is documented, and he wished to lead, as did a man who never made it to Officer class and spent his life teaching students and was a known quitter, jobs and marriage, so may well bail-out on 1079 and persuade some of the others to do the same and make a snow den.
I think the sole exit, the tent flap end, was likely blocked by the others, either accidentally or deliberately, and so some mutineers cut their way out. They were not allowed to take anything which helped insulate the tent, including skis as these lined the floor. The damage to the tent, the adrenaline of the stand-off, and the angry recriminations which followed, and the young men in the tent having expended so much energy in digging the trench, all led to a gradual hypothermic situation IMO, and the disorientation that can involve.
While remaining at the tent the other 5 would be worried the mutineers would take everything from the cache and follow their own tracks back to Vizhay, snow denning along the way.
The tent group then descended the mountain, most likely at Zina's insistence when Yuri Doro started to show the effects of the cold. They did not take wood cutting tools as they planned to make their own den, but at the foot of 1079 Plan B, a fire, took over when the 2 Yuri's were affected with frostbite and so 3 had to make a den for 5. They had matches in their pockets to keep them dry.
When the fire failed to save the Yuris, who burned themselves while semi-conscious or fitting, the remaining 3 of Igor's group probably attacked the others in the small den in order to secure it for themselves as the only survivor resource. Logs would be taken from the fire and 4 people blinded by torches. This may even be what some of Semyon's photo's, taken on the camera he took from the tent, represent, taken during an earlier scouting mission to the den, the position found by the snow spoil.
Three men were hit on the head to render them unconscious and they were dragged to the ravine, their bodies falling without any attempt to break their fall, so no broken collar bones or hand injuries and the impacts absorbed by chest walls. Lyuda was straddled and thumbs pressed into her throat until she passed out, kicking and scratching., causing Zina's baton bruise and the scratches to Igor's underarm. She came round in the ravine and suffered a second fall when she tried to climb out in the dark. She heard Tibo groan and placed her hat on him.
The unique thing about this mystery is we instantly know so much about the hikers that we cannot imagine them doing anything bad to each other, even if they become split into two adversarial groups and the human survival instinct, that panic, kicked in. But what probably happened was there in plain sight all along, and even alluded to in the autopsies where the pathologist distinguished between the ravine 4 dying 'due to violence' and the others 'a violent death'. One at the hands of others, the rest due to Mother Nature.