@DAN_THE_DANDAN, Ah I see. For me I only saw two ways of going about it, a controlled fire or setting the fire on the roof and then pulling the fire alarm and running downstairs yelling fire.
I think you greatly underestimate the disaster preparedness of the mall's employees, either that or I'm overestimating them. The Fire Department regularly inspects buildings in their jurisdiction and make sure that all civil codes relating to fire are being followed, warning and exit signs, evacuation routes, sprinkler systems, and other necessities. You can't open a public building for business unless the chief of the fire department signs off on it. In addition, the company that owns the building likely conducts fire drills on either a quarterly or twice yearly basis.
Now I don't think fire drills and evacuation training are required by law, but most companies do have them so that they can avoid lawsuits and bad publicity. It'd be really bad for a company if a fire breaks out and lots of customers die.
If the maid actually set a fire in the mall, the fire alarm would sound and sprinklers would activate. Now because of the sprinklers, the maid will need to use some type of accelerant in order to keep the fire going. Once the fire alarm detects the fire, the mall employees will start evacuation procedures and check if it's a false alarm. If the fire is set in the right place camera evidence will confirm there's a fire. Warnings to evacuate would be delivered over the intercom system while store employees would begin guiding customers onto evacuation routes. Security guards would come out to maintain order and prevent a panicked rush out of the building. The fire department would be on the way the very moment the fire alarm sounded. People would be evacuated and then the building would shortly collapse soon after.
The biggest risk in this scenario is to the firefighters who may arrive before the building collapses and after the evacuation. Though I would call those acceptable losses, it's not as if threatening with guns would be less risky as instead of firefighters, police would be on the scene especially if no hostages are taken. Though I imagine the author somehow works around this possibility or more likely the cops were too slow to arrive to the scene on time to enter the building.
The second biggest risk would be handicapped and immobile people that are unable to evacuate the building quickly, but even in that case, going in guns blazing is not going to get that elderly person who can barely walk or a guy with a broken foot or someone who is mobility impaired out of the building any faster than a fire would. However, in the case of a fire, an orderly evacuation is possible, which is one disadvantage of the guns blazing method, orderly evacuation is difficult in the face of actual gunfire. So most likely if things go according to plan the security guards would be able to assist the elderly and handicapped to evacuate the building before the fire reaches them.
Also, although the manager is a scumbag, you seem to assume that the security team would also be full of scumbags who'd just follow orders and put customers at risk even when there's clearly a fire. I disagree with that, just because the guy in charge is a scumbag that doesn't mean everyone under him is too.
I don't get the secluded room part, even if you set fire to a secluded room the FIRE WILL SPREAD. The only way to prevent a fire from spreading would be to start a low-temperature fire in a room full of fire-resistant materials. I'm not sure if you have experience putting out fires, but if the security team took even five minutes to respond to a room fire, fire extinguishers will not be enough.
I will repeat this, buildings must pass fire inspections before being opened to the public. So even if security doesn't see the fire on their CCTV, they will hear the fire alarm. You have fire alarms in your house, the store has fire alarms, the school has fire alarms, the restaurants have fire alarms, and there are fire departments scattered around the city because a fire can burn down a whole city. (Although that's unlikely nowadays, given modernized fire practices and urban planning)
It's true that this is set in South Korea, so the laws may be different, but I am highly doubtful they have less stringent fire laws. Given that they are a first-world country.