Let's look more closely:
NRS 200.160 Additional cases of justifiable homicide. HOMICIDE [the taking of human life by another human, intentionally or as a result of intentional action which a reasonable person would assume is likely to take life -- this section implies also the threat of such action] is also justifiable when committed:
1. In the LAWFUL DEFENSE [not while committing a crime] [. . .] of ANY OTHER PERSON [whether or not you know the person you are defending], in his or her presence or company, when there is REASONABLE GROUND TO APPREHEND A DESIGN [if a reasonable person would believe it] on the part of the person slain to COMMIT A FELONY [there are a lot of felonies, from rape to burglary to robbery to arson of occupied buildings] OR to do some GREAT PERSONAL INJURY [you don't have to know if the beating is of the level of a felony] to the slayer or to any such person, AND there is IMMINENT DANGER [means, opportunity and apparent intent] of such design being accomplished . . .
In other words, yes, you can use deadly force to defend the security guard against a beating, BUT when the danger has passed, the justification expires.