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He pushed a trolley with 40 litres of gasoline to the entrance and lit it. That would happen too fast for a security guard to prevent even if he were inside and paying attention. Looking at the floor plan, there was only one exit on the ground level, and possibly one emergency exit in the back (which he might have blocked.)

They just needed a building designed for fire safety; they've needed that every since the last arson attacks killed 40 people in Tokyo. (Authorities did nothing to change the regulations, 'cuz it'd hurt profits.)

So more fire exits and maybe some fire escapes from the roof. Also properly ventilated buildings that dont suffocate everyone when the smoke rises to a blocked exit at the top of the 3rd floor. And they needed multiple properly labeled doors that open outward to the roof, which would be equipped with a ladder. 

Here is their floorplan:
https://www3.nhk.or.jp/news/html/20190718/k10011997791000.html?utm_int=detail_contents_news-related_001