Does Pierre stay dead if I kill him?
Solution 1:
Death is a temporary state in Monaco.
As you mentioned, guards can revive other guards, just as players can revive other players in multiplayer mode. Even in singleplayer mode, if all four characters die, "justice is served", which means they get arrested, not killed (there is no death penalty in Monaco, since its abolition in 1962, the year of the last exectution was 1847).