[How stupid can they be? What nut job would store that much explosive in a population center?]
Very few laypeople make the connection between fertilizer and explosives. And that’s likely what was at work here. (The Haber process, which revolutionized the production of fertilizer, also had this effect on the production of explosives).
The US, despite (1) a globe-spanning war that saw widespread use of all kinds of volatile chemicals, (2) a massive governmental apparatus that sprang up to fight that war and (3) a general reputation for competence, failed to foresee the Texas disaster. There was no hope that Lebanon, with its mishmash of literally warring tribes, ethnicities and religions, could have foreseen this. Everyone’s too consumed in the day-to-day business of trying not to be stomped by the various protection rackets at work there for anyone to be thinking about the big picture (i.e. things like chemicals, the appropriate storage facilities and environment and a routine inspection regime).
“Very few laypeople make the connection between fertilizer and explosives. And that’s likely what was at work here.”
This is the MIDDLE EAST we’re talking about. They may not be a bunch of Nobel winners, but they sure as hell understand explosives and munitions.