I actually don't mind killing PCs with some of the above here - not saying that I seek out opportunities, but I've played with squishy casters who are bold and/or dumb enough to wade into the enemy's back line to take advantage of short range AoEs like Burning Hands and Thunderwave, and you better believe their response is to beat the ever-loving shit out of that caster so they don't get up and do it again. And past the opportunities that cocksure players give you, it is 100% okay if a character dies, even one that a player has invested in - adventuring is dangerous and combat is especially brutal; the dragon's not going to reposition themselves to exclude a downed PC from their breath attack, the vampire's not going to pass up an easy meal from an incapacitated caster. If your games are going to be impactful and climactic, the stakes need to be real, and you can't pull any punches.

But there is an important caveat in all this - what's not okay is trivializing PC deaths, whether they died through pure chance, or wildly unbalanced encounters that end in TPKs, because that (especially the latter) ruins games and creates players who invest nothing in their characters, or worse, start to see everything as a numbers game and work to build the murderhoboiest character they possibly can. If a PC dies, it needs to be a scene. After combat's over, make a point of narrating the aftermath. Give the PC last rites, have the surviving members of the party talk about their favorite moments. Some of the best, most heart-wrenching sessions I've run are the ones where a character dies.