The only system I've ever seen in which the GMs doesn't have full control over a game's lethality would be Original Traveller, in which a PC could die during character generation (which, BTW, happened to my first OT PC) due to a die roll the player chooses to make.
In all other systems, even if their combat mechanics skew towards hyper-lethality, the GM can still emphasize role-play over combat. His NPCs might tend towards being gun-shy, favoring threat and innuendo over actual combat...after all, they "know" just as well as the PCs how dangerous the "world" is.
Conversely, a "Killer GM" can turn "Teletubbies: the RPG" into a bloodbath if he so wants.