It makes some sense, he lost his head. Remember he wasn't fighting this for practical reasons, he was fighting it out of emotion. Given how deeply he feels about his sister, I can see how in the heat of the moment, after the fight of his life with adrenaline pumping through him in addition to unfathomable rage, he lost his edge and made a stupid mistake.
Honestly his only real mistake was standing on top of the mountain, in ripe position for a reversal. Given the afore-mentioned factors clouding his judgement, he probably made a snap decision that the mountain was down for the count, as he had been lying on his back for some time. Any other man probably would have been down for the count, and correspondingly his instincts overrode his reason.