This is where context matters. Are the PCs required to do this, or are they doing something stupid? Could they avoid the danger by doing something obvious like roping themselves together? Some DMs are more willing to kill PCs if they are doing something stupid.
I've killed a few PCs in my time.
IMO there's a difference between setting up a situation where a PC could potentially die (and then it happens) vs. unilaterally killing a PC.
For example, setting up a 200' cliff scenario (so 20d6 damage on a fall) and the average PC currently has 40hps. falling could well kill a PC. The PCs know this and should try to avoid it happening.
Vs. Just "a cliff" with the DM being cagey/uninformative as to the depth (even roughly) and when a PC tumbles off the DM just looks at the player and says "Sorry, Rothgar's dead." That's the kind of stunt that was done in old school Tomb of Horrors style adventures - and I'm not a fan.