D&D 5E (2014) CR and Monsters: Let me get this straight...

[Boarstorm]

Thanks, I skimmed over that bit.

 

[Agamon]

One could read this as "this is when a monster won't kill you" or "this is when a monster won't kill you outright". I'm ok with the former, but worried about the latter.

It's more art than science. The CR is a guideline. Any monster could kill anyone (okay, a kobold killing Lvl20 fighter is unlikely). The CR just tells you what to expect. I've had a Lvl 2 PC killed outright by a CR 1 creature in our game. That's the game, dice are the final arbiter.