Now, to clarify, we're talking about PC protagonists here, which means we
1) aren't talking about PCs whose existence (including death) is made to fit certain parameters of the players' fantasy (go back to read my theoretical LOTR RPG design posts)
2) aren't talking about PCs that are gamepieces who die simply if we don't play hard enough
3) are talking about passionate characters who carry a story by moving through situations and their resolutions
To specify further, I intend the game to run primarily as a structured freeform, with mechanics looted from other games applied and evolved strictly as needed, when needed. I really want to fit the game to my players while retaining the goals of play I'm aiming for.
All that said, there can be a bunch of safety nets installed between a PC and death, I'm going to list a few.
OOC Principled Solutions
One: Nobody but you can say what happens to a character you own. They can say their characters do horrible things to him, but what happens is not in their authority.
Two: Declarations of outright murderous attempts are understood primarily as intent, not perfect execution.
Three: a possibly controversial premise: killing as the only option of solving a conflict is baggage from other games. Characters need only disable, disarm, manipulate, intimidate, get out of the way, defeat, knock out (etc.) not necessarily murder.
These work together to soften the general attitude towards killing, while retaining all the brutality. Tell me if you need concrete examples.
1:Murder is the absolutely last resort that carries tremendous consequences in many instances. Are you sure you're doing this?
2: It's harder to kill a person than you think. Most available poisons need weeks of constant application to destroy someone. Combat usually ends with wounds, surrenders etc. Even when mortally wounded people tend to keep going for a while.
Protagonist PCs and Non-Protagonist PCs
As a general rule Protagonists don't die in the middle of the movie. If they do, they'd best come back as a ghost or something. They do die at the ends of movies sometimes, as a statement. The death of a protagonist is not meaningless, it closes an arc, a particular story, it's part of resolution.
Non-protagonists die a lot. Sometimes they're just redshirts who get popped a plenty, faceless hordes of bodies. Often they die to make a point, to show just how ruthless this Antagonist is, or to inspire the Protagonist with their sacrifice or they become a sacrifice the Protagonist makes.
In the style of Ars Magica or early D&D or even, say, Montsegur 1244, players get to play both. Character monogamy is not the default assumption. The players, together with the dice, decide who the protagonists are: non-protagonists die early. Protagonists stop being protagonists when they become boring, non-conflicted or dead.
In short: Protagonist death is possible but neither random nor easy, N-P death can be random and easy. N-Ps can be elevated to P status and vice-versa. The mechanics (if any) should reflect that.
You'd never want your protagonist to get randomly decapitated because of a bad die roll. That doesn't mean you don't want to see it happen in the game (to someone else). There's no reason why protagonists and others should share the same (unfortunate) rules/fate/mechanics.
This is probably all very abstract, but I find it difficult to spell it out in plain language at this point. Also, it's not definitive, it's just options and possible tools. Concrete examples would probably help but I didn't want to make the post too long (an usual foible of mine).
The point is, by setting all these principles, assumptions and expectations about the game (without doing anything mechanical about it!) we've already established a bunch of safety nets that prevents the characters from "just dying" (something we don't want) without removing the actual threat/possibility of death (what makes play fun). All part of the system.