Doesn't the red halo appear to whoever got the person's life down to that state first? Or at least that's what I've heard so as to give the person who did the deed first opportunity.
I swear I've been ready to save people on multiple occasion, almost even spamming the grapple, but the enemy seems to always get it first.
You don't see the halo until it's too late. Kind of like grappling sprinters half the time. If I save anyone it's by watching their health, and hitting R1 as I know they're about to go down. You can definitely beat out the brutal kill that way. If you're waiting to see a red halo, odds are the enemy has already initiated the brutal kill and the game's still catching up.
From my experience it operates the way you said, the enemy sees the halo first, then eventually it opens up to everyone - just like how people steal kills. You have a short window to execute your brutal kill, if you don't (because you're recovering or finishing a move), it opens up to other people.
No one can steal a kill from you simply by hitting R1 the same time as you if you just initiated it. There's a definite delay before it goes global. So team saves seem to do the same thing. Like i said, watch their health, you know when they've almost had it.
She's going to have to fall on her sword. Which means that we have to stick one in the ground, trip her onto it and get someone to jump up and down on her back for ten minutes. --
Malcolm Tucker. Modern day saint.