If the security system requires people to close the door on somebody trying to follow them, then the security system is at fault, not the employee.
A proper system allows only one person per authorization, using a two-door system, a turnstyle or a manned entrance. Having employees punch a code and then tasking each employee with supervising the open door means they had no security at all. This is just an annoyance to the staff, and is easily defeated.
I believe one of the victims was at the door entrance. May have been forced entry.