Just a quick note to point out that in the real world, it's fairly difficult to kill a human in less than around 10 seconds other than "massive damage to the head and upper spine". Basically, you only have two physically realistic causes of fairly sudden death: the brain runs out of the chemicals it needs to function (oxygen, glucose, etc.), vs the brain being massively disrupted by physical or energy trauma.
IIRC modern military field stations have something like a 98% survival rate if the patient reaches them with blood still circulating to the brain, including via CPR in the helicopter. From a fiction standpoint, even decapitation doesn't kill the head (brain) instantly, which is relevant in situations involving cyberware, magic, curses, divine intervention, etc. where the head can still do things you don't want even if the body isn't connected anymore.
Loss of consciousness in around 5-10 seconds tends to be used as the benchmark for "brain suddenly deprived of oxygen"; decapitation, vacuum exposure, ripping the heart out of someone's chest, etc. where you don't even get the benefit of the remaining oxygen circulating in the blood. (As compared to breath holding, for instance, which can go on considerably longer. Note that it's basically impossible to hold your breath in a vacuum, and the lungs start acting in reverse, to pull oxygen *out* of your blood.) Even then, permanent death doesn't occur until some later point, fairly ill-defined but probably tens of seconds more.
Of course, sufficient shock trauma can render those transition seconds un-useful for the target. If you're trying to kill someone so fast that they can't respond, and don't have available means to just blow their head off, blunt trauma to the head will probably have them stunned / concussed until the rest of the damage catches up to them.
In short, when you're talking about a game like old D&D or fortress mode DF, where an action is tens of seconds to a minute or few, "instant" death is fairly easy. Once you get into games like GURPS or adventure mode DF, with second-by-second action, it's really hard to kill someone instantly.