JadeKaiser it's an emotional state that triggers the Mangekyo Sharingan. Not killing someone. The absolute despair and grief that comes from seeing a close friend/family member/lover die right in front of them is what triggers it. Upon feeling that a special chakra is released that changes their eyes. There's a reason why sasuke didn't awaken his until after he learned the truth behind itachi. If it worked like you claimed it did sasukes would have awoken the moment itachi died.
[Citation Needed]
Even if it
did wait to be unlocked until Sasuke found out the truth, and I don't recall there being any actual proof one way or the other on that, that doesn't mean what you say it does. Sasuke
did kill Itachi after all, as I explained above. It would just mean that a sharingan user has to
know that the person they killed was someone close enough to them to qualify before it would work, but that finding out later is perfectly acceptable by whatever standards the sharingan uses.
Your explanation is acceptable with canon, but so is mine. Moreover, I prefer mine because otherwise you get people cheating the system to unlock their friend's M. sharingan by pretending to die in a training accident, as I have seen far too many fanfic authors abuse. Also, because mine is the one where we have a direct statement in canon to that effect, even if the other half of that statement ("...your closest friend") was shown to be a bit so-so on precise accuracy since Itachi was close enough to count for Sasuke.
I also prefer mine because yours screams of trying to find a purely biological excuse for it. The swirly eyes are not purely biological; there is a spiritual component to them as well. This is both shown and explicitly stated when Orochimaru first tried to possess Sasuke and had it turned around on him, and is also pretty much a requirement for some of the
other weird shit that Kishimoto built up around them.
Having them require that you
actually kill the person because magic eyeballs is perfectly in theme. Having them upgrade when your brain undergoes the precise emotional and hormonal cocktail that results from such an act not only isn't, but wouldn't work even if it was because different people would have different emotional reactions to such a thing and would have slightly different brain chemistry to begin with in the first place.
If it was triggered by a specific emotional state, some people might unlock it after they killed a random puppy by accident. Others wouldn't be able to unlock it even if they slaughtered their friends and family wholesale in a massive murder spree.