Well, I for one didn't look at if from a deontological point of view. Unlike other victims of coercion, murdered people tend not to make much of a fuss about having been forced to die. However, their death tends to produce sorrow and anger on those emotionally and economically reliant on them (including their creditors, superiors, subordinates, clients) and causes the loss of a hub of social network and a repository of knowledge and skill accumulated over a lifetime. In other words, murder, for a stable, sedentary, densely structured society, is extremely wasteful and troublesome. And it leads to a shitton of paperwork. That's for just one death.
However, from a Golden Rule/Reciprocal Altruism POV, committing murder is an extremely bad idea because, besides the aforementioned problems with each individual murder, the fact that people can get killed, for whatever reason, may make one fear for one's safety among fellow humans ,raising stress levels to presumably unbearable heights and cause a limitless waste in resources in personal security, . Think of Israel-Palestine, where people constantly live in fear of being killed by some crazy suicide bomber or some trigger-happy teenage soldier. Despite what the actual risk of that happening might be compared to the risk of dying in a traffic accident, much more money ends up being spent on this sort of thing than on road safety. Car accidents, as the Joker would put it, are "all part of the plan". I love how effectively that character uses The Dark Side and The Fallacy Of Grey. Does anyone know any examples of similarly eloquent villains? (Besides Nietzsche that is).
Hence why we are taught to heavily frown upon it.
A Mongol from Genghis Khan's Golden Horde might see things differently, is all I'm saying. There are also countries like Colombia where murder is extremely common, and often absolutely senseless. Society functions, people get used to the fear. Humans can get used to a lot.
Deliberately murdering a mass of people causes the same effect
To break up the awkward silence at the start of a recent Overcoming Bias meetup, I asked everyone present to tell their rationalist origin story - a key event or fact that played a role in their first beginning to aspire to rationality. This worked surprisingly well (and I would recommend it for future meetups).
I think I've already told enough of my own origin story on Overcoming Bias: how I was digging in my parents' yard as a kid and found a tarnished silver amulet inscribed with Bayes's Theorem, and how I wore it to bed that night and dreamed of a woman in white, holding an ancient leather-bound book called Judgment Under Uncertainty: Heuristics and Biases (eds. D. Kahneman, P. Slovic, and A. Tversky, 1982)... but there's no need to go into that again.
So, seriously... how did you originally go down that road?
Added: For some odd reason, many of the commenters here seem to have had a single experience in common - namely, at some point, encountering Overcoming Bias... But I'm especially interested in what it takes to get the transition started - crossing the first divide. This would be very valuable knowledge if it can be generalized. If that did happen at OB, please try to specify what was the crucial "Aha!" insight (down to the specific post if possible).