When I couldn't divorce my emotions from my screen, I was a literal teenager. The peaks of my RP were higher and the valleys were much lower, and it is, all in all, not an experience I would ever care to repeat. This is not a dig at you, @The-Tree-of-Woe, but I wonder why most people can't?
Surely the ability to take a step back from everything and get your bearings, to encourage healthy emotional separation and healthy emotional investment, is something MU players should encourage? Frankly, I wish someone had said as much to me back when I was that teenager, before I said and did things that absolutely ruined friendships -- even when, looking back on it, I can still acknowledge that my emotional response was understandable and not unexpected.
It's a rough thing to say but, again, let's be honest here: not everyone's emotional reaction is proper. The person who is upset because a character they liked to spend time with is dead, and they need a moment, okay, fine. Understandable.
The player who has an extreme emotional reaction and complains to staff about the character being killed and disrupts the scene to try and force an OOC retcon... Well, something something "no right to demand", "goalposts", etc.