I am a horrible D&D player.
Picture it: I have a little gnome fighter. We try to fight off a Wight without magic weapons. Eventually, we douse it with oil and light it on fire, but I'm reduced to 0 HP. So is one other guy.
They stick me in the corner as we try to find our way out of a certain deathtrap room.
And out pops another Wight!
OH we are SCREWED!
Then I remember, I have a necklace of fireballs with one fireball left. I use it. And predictably, four of us end up at negative HP. The cleric is irretrevably dead. We are able to survive because we had a bunch of potion. But it was a mess, and a preventable one, too.
Picture it: We're creeping up into a completely pitch-dark passage. My druid and the cleric volunteer to follow our guide in even though our infravisions aren't working. At a certain point, the guide demonstrably huges the wall. We thought it was to let us pass.
And a green slime falls on the cleric! He loses his plate armor, and almost dies!!
I should have seen it coming - the guide knew what was coming.
Picture it: I've taken over my daughter's magic-user for the night since she can't come play. He is only level 4 so he doesn't have anything that could really kill us. I used Sleep a couple times to great effect.
Aha, treasure! A Ring of Spell Storing that has one instance of Lightning Bolt in it!
We fight a statue that can shoot lava and it's immune to +1 or worse weapons. So I line up the shot really carefully, so the lightning bolt only hits the statue.
Bam! It explodes!
AND SHOWERS US WITH LAVA!!!
The DM rolled max damage and everybody but one person missed their save. And my daughter's wizard was fried instantly.
BUT IT'S NOT JUST D&D!!!
Years ago, our Champions group was in an enemy moon base. My archer shot off a Darkness arrow, blanking everyone's vision except one person. Then he shot off an Explosion EB arrow, and put a big hole in the station's wall, killing everyone.
by the way, this is all perfectly OK in a vacuum.
If everyone is fine with regular player deaths, then a killer player is not that much worse than a killer dungeon. But it can get really tiresome for the other players if they're NOT cool with dying every third adventure.
I'm a good DM. I don't kill my players. But the dice sometimes do. As it should be!