I'm not the GM so I have no say in how these go down
I would like to question this. You certainly have a say in this. If it's something you don't like (which seems to be the case), you can always bring it up and start a discussion about how you want to deal with this in the future. Having every opponent be a potential death curse that kills me if I don't have a ritual prepared to take care of it feels incredibly broken in the first place.
And as others have said, a death curse on top of a taken out result shouldn't work. You're out of the conflict, you can't do anything anymore. A concession would work, sacrifice yourself for the greater good, but as part of a taken out it feels like cheating. How did they go down in your game?
Mostly I wanted to know was what is your take on non-mortal death curses? I kind of feel that if red court vampire spellcasters could death curse the war would have gone differently, that said wardens are notoriously good at dealing with death curses, so I dunno.
For one, they don't have too many of them. Some, yes, but not too many of them. Reds are dangerous enough as they are.
And taking for granted that they are skilled enough, they would still need to have the presence of mind to actually throw it. That's not really on a predator's mind. Especially on an immortal predator's, as they are so set in their ways and convinced they will go on forever. Death and thus a death curse isn't something they are going to be thinking about.
And remember how Harry was preparing himself for his death curse? It takes at least some time to gather yourself and cast it. In the heat of a battle, that might not work too well. Humans have the opportunity of a more flexible mind. We can, if need be, spin on a dime and choose death over our survival instincts. It's one of the big themes of the Dresden Files, really, choice vs. instinct/nature.
How exactly you implement that is up to you and your group, of course.