There were lots of strange things that happened that day (and following days).
I was trading in the city and actually had two brokers on the voice box in the twin towers. It was eerie the way it went from one minute shouting prices to the next minute just white noise. Sadly, they both died.
We then went into the CEO’s office and watched it in real time on his huge (at the time) TV. I was totally shocked when the towers collapsed. That shouldn’t have happened. It subsequently turned out that it was due to fraudulently not using the right quality and quantity of building materials.
Then there was the bizarre fact that 9 private Saudi jets were able to fly, even with all other planes grounded, some with members of Bin Laden’s family. There has been no explanation to date of who authorised these flights or why.
There was an assumption, before the shoe bomber and 9/11 that hijackers didn’t want to die themselves, so as long as the hijacker got on the plane with their luggage, the flight was safe. Sadly, with suicide bombers, that assumption had to be changed.
Finally, as to the OP’s question, I think we all tolerate strangers dying for our leisure. Controversially, every time we fly, we contribute to global warming which, statistically is contributing to pressure on the whole system and causing random deaths. I know that feels very different, but it you didn’t know any of the 5,000 odd people who died in 9/11, why would you care more about them than those who died in tsunamis, earthquakes, wars etc. Humans are strange; we care about people we feel are geographically close to us or ‘like us’ far more than others.