The two sentences are grammatically correct, but pointless. They explain nothing of what the guard actually does, because you have used the verb-form to define what the noun-form does. It is unlikely to make any difference to the degree of pointlessness if you add 'security' to these sentences.
If you want to write a worthwhile sentence, you have to provide information or opinion. Depending on your viewpoint, you could say:
Security guards generally sit around looking bored.
Security guards act as a visible deterrent to anybody thinking of breaking the law.
Security guards provide a first-level response to emergency situations such as fire, accident, illness or criminal activity.