I think I have the answer to what I think you're asking.
Why carry in "DA-first-shot, SA-all-follow-up-shots" mode?
Good question, and I believe the answer is:
Because of the adrenaline factor mentioned when the SHTF, and you are actually aiming at a BG but not fully sure whether firing is warranted (such as when an LEO holds a suspect at gunpoint or when a civilian is pointing at a contact-weapon-wielding BG who is beyond 21 feet but possibly closing), one wants a longer, safer, DA trigger for that first shot, to minimize the chance of an AD.
BUT, once the first shot has been sent flying by a DA pull, then that means you have made the conscious decision to shoot to protect your life (or the life of another), and therefore you will want to incapacitate the aggressor. How does one do this? Shooting as rapidly as possible until the aggressor goes DOWN - how does one do this? - short SA pulls will help. No need to worry about safety at that point - in fact, you want to be as "unsafe" as possible towards the violence-threatening aggressor once you have made that deadly decision.
Long answer to simple concept, I know, but I demonstrated the "shooting justification" aspects as well.
PS. Don't EVER try to manually lower the hammer on a live round, IMO, unless it's a revolver and you have no other choice, in order to unload. If it's an SA-only, rack that baby to eject for cleaning, or carry cocked and locked. If you must carry with the hammer down, then I would suggest empty chamber carry is the best, IMO.