I always thought that the problem with mass murder was about the same as the problem with normal murder except multipled by a thousand or eleven million or however many victims there are.
For a strict utilitarian, the main problem with non-torture murder can be seen as the fear it produces in the population.
Strict utilitarianism is quite common here, so guessing that wedrifid is one isn't that much of a reach.
I don't think "strict utilitarianism" refers to a specific, well-defined moral system, but my point still stands if you're referring to the general class of moral systems and methods of moral reasoning that are popular here; involuntary death is bad, whether torturous or not, so killing ten million people is at least as bad as the sum of the individual badness of killing each of them. The "at least" part is to take into account any further negative effects of mass murder, such as the one that you mentioned, but compared to millions of people dying involuntarily, I really doubt that's the dominating factor.
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).