After this long with the mental link, Nathan could pick out the wild, instinctive thoughts that Peter was usually able to mute or keep under wraps when he had full control. These were those, the images that came with that panic, and he knew, by the content of them, that Peter would have never shown them otherwise. He just held on to his brother all the tighter for it, as there wasn't much he could do for any of that right now, other than to get him out of this place.
At Sarah's order, there was a seconds long debate about leaving her, and Faith as well, but this had been one of the contingency plans and they all knew the goal here – to get Peter out. If reinforcements were coming, they had to act now. Raising his gun defensively, Nathan shifted his grip on Peter to bear most of his brother's weight – carry him, if necessary – and pulled him toward the stairs. He could tell the idea of letting go of Sarah, and more, of leaving her, was, to put it mildly, wholly disliked by Peter, but the potential for being the one Peter was angry at was something Nathan was willing to accept if it meant getting his brother out of this place.
They only made it one flight before the stairwell was no longer empty, more human and vampiric employees blocking their way just as Bela had predicted. Leaving the vampires for Faith to deal with, Nathan raised the gun in his hand and fired at the human half, two bodies toppling and leaving the others to scramble back to avoid falling down the stairs as well. It wasn't flames and it wasn't equal payment for the things done, but it was effective and necessary. Another three bullets, one a missed shot when the target was just slightly quicker the first time, and two more people were down, one dead, the other not getting back up now with a blown-out knee.
The path cleared that much, Nathan carried Peter down the stairs. It was too closed in, too many angles and with too much potential to be hit by ricocheting gunfire to fly in the stairwell, but adrenaline did its job and Nathan descended with speed, Peter's weight negligible.