Death to a guardian must be like when Dr. Strange was on a time loop with Dormammu, (Recent Movie) dying in different ways, but still coming back with a corrected time frame and perspective of the outcome. Imho, if a guardian was already dead, why should he even care to fight for a cause that will not benefit the dead(which we already "were)? After everything is all set and done, the Traveler will awake and we'll all end in the same place Ghost found us in; a fish tank. If you think about it, guardian's are nothing more but fourth dimensional projections of their own cognitive existence(That's why guardians need a certain ghost to survive. The light, is the power source, and the guardian is a program of ghost). In the first mission there is somewhat evidence of this. Ghost dissolving us to enter the ship by the old hanger is one, then when ghost saves us from getting blown to bits by Oryx when we try and infiltrate the Dreadnaut is another. I don't think Guardians care to much if they live or die. They have nothing to lose..