If I was the clerk, I’d leave and lock the little bastards inside, then call the cops. They might break the windows to get out, and hopefully hurt themselves in the attempt. But it would take a lot longer for them to get out, and perhaps the cops would arrive to arrest those still stuck inside. Yeah, I’d probably get arrested for violating a city code or law, but who cares. Because of my years of dealing with security, my first thought is to contain the crime scene and those involved.
One of these days, a clerk will go "Death Wish" on them, using the gas available outside to burn them after locking them inside. Someday.