Not sure what you're saying Pex. "Killer DM" usually means acting in bad faith or power tripping. That's different from creating oppositional scenarios where the PCs have to play well to survive, which is the premise of the game.

Yes, frequent deaths can mean there's a mismatch between GM and player expectations or skill level. But it can also just reflect a high "difficulty setting", which everyone may be on board with.

When is it too hard? When the players aren't doing anything wrong yet are dying so much that it becomes irritating. Where that threshold is will differ between groups. It seems like you have an instinctive sense of where that is for you but that doesn't mean other DMs are "tyrant DMs". Especially if you're playing 5e, where death is less punitive than ever before.