One of them is dying slowly and painfully, so the other has to kill them so they can effectively sleep through the worst of the dying/healing process (ie your body is metabolizing some poison/regrowing a limb/etc. so I'm going to kill you quickly so that by the time you come back to life you'll be mostly healed again. Could be the first time or something they've had to do before, could be something sad but not overly upsetting, could be something hugely triggering (remembering killing each other during the crusades, worrying about the other not waking up, so on and so forth). Bonus for the injured party delirious with pain and begging for help!