It's hard to appreciate the sheer number of people that exist in the world... Surely there are enough crazy people to populate an entire city.
Is it possible that intelligence is inversely correlated with insanity? I thought it was the opposite.
This behaviour seems only steered away through fear of getting caught. If there is little fear of being caught or of being prosecuted, then this behaviour will be as rampant as it can be without calling too much attention - or those like journalists who do call attention to it will also make the list to be assassinated/murdered in the most extreme cases. There are more covert things people could do to simply ruin someone's life by doing actions indirectly.
If you restrict that to even 1 in 20 violent, then you're talking 400 people world wide.
Factor out kids, elderly, etc and you're talking 200 really scary motherfuckers. Figure 75% are employed by governments, and you're talking 50 rogue badasses -- which seems vaguely in line with the number of really dangerous terrorists, paramilitaries, etc.
My guess is that the number of people that are both interested and capable of the things you propose are busy making tons of cash on drugs, terrorist violence elsewhere, etc.
But they're reasonable bounds -- the number of terrorists are very low, we know this for a fact. So we should suspect that the contributing traits are relatively rare. And if we want to talk about people who really scare us, then it should be people who are real outliers.
An NFL player is a one in a few hundred thousand, and an NFL star is around one in a million. So if we're looking for the Tom Brady of terrorists, it would make sense that we don't want a fraction substantially higher than the number of Tom Brady's in football players.
So I cut off a segment of the population that would have the traits required and estimated how large it was -- 99.9th percentile capable, 99.9th percentile crazy, 95th percentile violent. (Intuitively -- if the smartest and craziest kid in your high school were the same kid (1 in 1000 each trait), then you picked the most violent in 20 of those kids.)
This turns out to be ~400 people, which seems ballpark right. I'd certainly expect the answer to be in the 120-1,200 person range.
As long as your estimates aren't too far off and you aren't systemically wrong (eg, always rounding down), estimating is usually a reasonably accurate method to get a feel for the scale of something.
I was doing homework one night and felt the house shake, I thought it was an earthquake. It turns out a bomb went off in a 5 story apartment building nearby. Someone was trying to kill a banker so they left a bomb by their door.