It now seems that Paddock shot the security guard before, not after he'd slaughtered the innocents.
The guard heard a drilling noise in Paddock's room and was then shot; immediately after this Paddock started on his killing spree.
This implies that he was forced to start shooting out the window(s) earlier than he'd planned, but then how much longer could he have waited?
Wasn't the concert almost over when he started killing people?
It just doesn't make sense, but then again nothing about the incident does.