Not to smash the whole gag a wizard thing but that doesn't work. In the books it is clearly explained that a wizard doesn't need to speak, wave hands, or anything to cast. These are just insulation against magic's effects on the caster. With words, gestures, and preperation the wizard only gets tired (ie. takes stress). In one of the books Harry is in a choke hold and blacking out, can't talk, move etc, and he casts a lightning bolt at the guy, it just affects him alot more making him see shades of the nevernever. Also, if I were a warlock and I was going to use a deathcurse I'm pretty sure it wouldn't matter to me if I had a headache at the end as I would be dead so gagging me would be silly.
Death curses require concentration and time to build. I've read all the books a few times and each time a deathcurse is used the caster has to draw in alot of power and it takes time. It was also clearly said that sometimes a death is so fast that a person can't cast the spell. Also you have to remember that even if a person could cast one, is the last thought through your mind going to be "hmm, I'll use that death curse now! BLAM!" or "Oh god oh god that guy just stabbed me/This can't be happening". Warlocks are mostly the untrained that stumble into their power and aren't taught how to use it. It probably never occurs to most warlocks that the possiblity to use that type of fuel even exists.
All this being said, NPC's do have death curses. It is up to your game master to regulate when one is appropriate, and what it does ultimately. A PC or NPC may even have access to one but may not be able to come up with the way to use it when the time comes. My game is bi-weekly and I can honestly say that it takes me the full two weeks to come up with one that will be truly terrifying to an enemy and that is tailored to that specific enemy! If you had as little time as 15-30 secs could you come up with a deathcurse or would you just try to blast the guy? Also, timing would have to come into it. What happens if your not as hurt as you think you are. PC's and NPC's dont' know jack about stress and consequences. The player might but your supposed to be pretending to be that guy, he doesn't have a sheet to say "Hrmm, I'm almost full on stress and consequences, this next hit will kill me, I'm gonna deathcurse!" If I were a wizard I would be very afraid to use that lest I not be as bad off as I thought. "Well, Bob had a papercut with lemon juice on it and would have made it but he fired off that deathcurse and died..." food for thought.
As far as executions go, the hood stops the curse. Deathcurses are most definitely evocation. That being said, you have to see the target to hit them with it. The stuff that you can do without L.O.S. is thaumaturgy and is far to time consuming for a deathcurse. First book Harry says that you have to be able to see someone to use the boom boom magic. Far as I'm concerned case closed. The only time a deathcurse should come in is during a fight.
I know I've gone on a bit here but in my game I am the warden. I've thought about this and other stuff like this alot.