As a DM, my goal isn't actually to kill characters. That is simply a side effect of other things that are happening (dice, monsters, player actions, etc).
This is something I sometimes struggle with... I want to accurately play the bad guys, and often their goal is to kill the PCs trying to foil their plans. However, if I put all my energy into killing the PCs (which is what the NPCs would do) then I look like a jerk. But I also don't want to give the PCs plot armor. It's a difficult balance to find, and sometimes a conflict of interest.
Ignoring the Head, why not the Eye or Hand?I'll never include the Eye, Hand, or Head of Vecna
Well it's just good planning. What, you expect them to do something crazy, like remove the periapt and stab them again?
Ignoring the Head, why not the Eye or Hand?
Hopefully, the assassin knows enough to check for and remove jewelry or other possible magic items after the second or third time they do an auto-crit sneak attack on the unconscious body and the target doesn't die. Or even research their target enough to know what protective magic items they have ahead of time...
Because if they just stand there sawing away at the unconscious guys throat...well, they deserve whatever happens.![]()

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.