There are only a few instances where I can see letting a character die in my game (this is my style there are others, but this is mine). If they have fought valiantly dispite obvious danger against an overwhelming force then it makes sense that they could and likely will die. If they are faced with a foe they have stuggled to face then dying when they face that foe is significant. If their death will save someone they care about, the rest of the party, the village, the world, they have decided their sacrifice matters, who am I to argue?
When I will strive not to kill my characters is when they fight a nameless, NPC or a wandering monster. If they confront such a danger when they are low on HP and resources and go for it anyway then they have decided to make their last stand, I did not decide it for them. They "earn it" by making choices that put them in mortal peril. To me that is a sliding scale and very situational. I find ratcheting up the tension does not require dealing out damage on every failed roll, only the ones that matter. It took me a while before I turned the dial to a place that worked for me and my groups. As they progress up in levels the dial will move but my aim will remain the same, if they die, make it count.