According to Bro Eli , he said it it the duty of a security officer to protect properties and lives at all cost and as well protect him/herself, which means use of weapons. As a security officer you are trained to weapons especially gun to fire at the enemy.
One can say it an error for a security guard to use weapon is when it uses it on innocent citizens, citizens that did not commit any crime. The duty of a security guard is not an easy one because you are meant to protect not only people's lives as well tha nation at large
A Security Guard using a weapons registered by the government that is licensed, has the right to use it anytime irrespective of it securing lives and properties which will not involve bloodshed of innocent ones and going against such law attracts consequences.