TLDR: For your own personal narrative, when does a model "die" in game and not just become too injured to fight on, knocked unconscious etc? My idea is only on failed armor saves of a 1. But what about against low AP weapons? Ideas?
Having been playing Sisters of Battle a little... too long, I've gotten to the point where I've named all of my squads and half their members. (In the Adepta Sororitas it works a little differently, squads are named after their leader, for example Dominion squad Bellatrix would be led by Dominion Superior Bellatrix) anyways I like to record their victories and failures in game.
A young Bellatrix's squad once charged an Ork Battle Wagon which she detonated with her Melta Gun before assaulting the horde of Lootaz inside. Bravely, her team slew the survivors of the burning wreck in close combat before turning to the next Battle Wagon rolling their way. While dodging bullets, Bellatrix and only one other remaining Sister, their weapons aimed true with the Emperor's glory and rendered this oncoming hulk into another burning heap... but the onslaught of fire from the Orks was too much and she sought cover, retreating nearby after her final companion was slain. Then, knocked unconscious from the sweeping tides of Orks, she was later found breathing on the battle field and returned to her convent by the surviving Sisters who honored her with the promotion to Sister Superior for her deeds that day.
So what is a reasonable mechanic for deciding when a model has actually been killed vs just too injured to fight on? My thought is only on a failed armor save of a 1, but what about against weapons that allow no armor save like a Lascannon? Even a toppling blow from such a powerful weapon might only maim it's victim, who with the proper medical care, could live to fight another day... just not that day.
Any good ideas? Perhaps an out of game "death" save based on toughness?