Holy hell. The synopsis for this manga doesn't fuck around. That incident on the last pages did affect me a bit. I'm OK though so that's great.
Anyway, why would this company or owner of this building hire a bunch of randos instead of hiring a security firm filled with professionals? You're going to pay these randos a lot of money for the work anyways. So why not use it to pay pros that actually know what they were doing?
Then again, this manga is called Ura Baito also known as shady part-time jobs. The owner of this building probably tries to hide something. Probably a dead body but who knows.