Pray tell, what exactly is the difference between the following two scenarios:
>Players offend high level assassin.
>Assassin is way more powerful than they are, has stealth skills that are so far beyond their perception abilities as to be essentially invisible, and has an opportunity to strike.
>He strikes, killing the PCs before they have time to notice what's going on.
>Players offend high level fighter
>Fighter is way more powerful than they are, has speed, strength, equipment, etc. so far beyond their own abilities as to be invincible, and has an opportunity to strike.
>He strikes, killing the PCs before they have a chance to react.
The choice is the essential part, not the rolling of the dice. Dice are just a way of navigating probability.
Why isn't it ok to "instantly murder" a player character? RPGs are a subset of games, and games at least games with skill involved, pretty much by definition, have an interaction of choice and consequence. Those choices will not all be equal, and in fact should not be, otherwise the act of making the choice does not lead to differing outcomes.
It should not be done arbitrarily, but yes, if the logical consequence of PC actions is "you die", they should die. It's no different if they stand in an erupting volcano, punch the king in the face in full view of the royal guard, or pick a fight with something colossally stronger than they are. If an enemy has motives, means, and opportunity to destroy the PCs, and no better use of the resources to do so, why aren't they employing them?