I'm not sure where you got the idea I think that every death is the DM's fault? I said this is a crutch for the specific situations when it is the DM's fault a PC will be killed. If a PC stays in the front line on low HP, it's absolutely not the DMs fault if they die, that was a decision the player made. If someone just has a really unlucky turn and gets critted into oblivion it might not be anybody's fault, just the luck (or lack thereof) of the dice. It's the DMs fault when the players are missing information that would let them make informed choices.

As for your second question, you could have it said by a character in game, but coming from a DM to a player it's a loaded question designed to stop them from taking actions. There are only 3 scenarios where it's appropriate to say seriously: firstly, when the player has too little information and is going to take an action without properly understanding the context. In this scenario it's more appropriate to ask what they think is going to happen or what they're trying to achieve to iron out the misunderstanding, rather than let them bumble about trying to work out what their mistake is.
Secondly, when a player is about to walk into some kind of hazard they had no way to know about or reason to think about - this is at worst gloating that they couldn't read your mind, and at best a sign you shouldn't have put the hazard there.
Thirdly, there are times when a player is fully aware of the situation and is taking a suicidal action - you don't need to ask if they're sure, they already decided. They might be intentionally trying to get the character killed, trying to have an epic character moment, or trying to set off a trap because the last 30 minutes have been pretty boring and they feel like throwing some excitement into the mix, but your warning won't change their decision, because it was made with that danger already in mind. There might be an underlying problem to deal with, but you're telling them something they already know.

Ultimately if you feel a need to say it you're better off thinking about or asking them about why they're doing it, and if it's not character shit solving the issue that caused it.