Here's an example, which I think is what Kodos the Executioner actually did. You have your world thats on the brink of starvation. They are all going to die if you don't find a solution. You could change farming techniques to increase food production, but that takes time. So the quickest solution, and therefor the logical choice, would be to kill half the populace. Just line them up and shoot every other one. That way demand for food is decreased immediately. Although your people might find it evil, you've just made a logical choice.
Well, not really. Unless the timescale worked out
just right, Kodos would have had to kill
all the people before there was hope of more food. Or then he wouldn't have to kill anybody, because people could easily survive with zero food for several weeks. And there is no indication in the episode that Kodos would have been facing the right sort of time window. Rather, he just made flimsy excuses for satisfying his murderous urges.
Timo Saloniemi