The thing is that people aren’t perfect rationalists, and part of being a good rationalist is acknowledging your own flaws and limitations.
If you accept to kill, you’ll kill, even in situations where killing wasn’t necessary, because you’ll stop searching the hypothesis space when you find a solution that involves killing. Or because you’ll estimate that killing one will save two, but your estimation was flawed—you killed one, and yet the two still die. And it’s also something you should know about the way humans work, that once you did something once, it’s easier to do it again—and the killing curse seems to model that quite well.
Harry putting himself a “I’ll not kill” rule is him forcing himself to find solutions that don’t require killing. Especially when you see how his “dark side” work, finding solutions to “impossible” problems when really pressured to do it, it doesn’t seem irrational from him to test the hypothesis that he, with his rationalist training, and his “dark side” creativity, can find solutions that don’t involve killing. And that only if that hypothesis is falsified, he’ll resort to killing.
The thing is that people aren’t perfect rationalists, and part of being a good rationalist is acknowledging your own flaws and limitations.
If you accept to kill, you’ll kill, even in situations where killing wasn’t necessary, because you’ll stop searching the hypothesis space when you find a solution that involves killing. Or because you’ll estimate that killing one will save two, but your estimation was flawed—you killed one, and yet the two still die. And it’s also something you should know about the way humans work, that once you did something once, it’s easier to do it again—and the killing curse seems to model that quite well.
Harry putting himself a “I’ll not kill” rule is him forcing himself to find solutions that don’t require killing. Especially when you see how his “dark side” work, finding solutions to “impossible” problems when really pressured to do it, it doesn’t seem irrational from him to test the hypothesis that he, with his rationalist training, and his “dark side” creativity, can find solutions that don’t involve killing. And that only if that hypothesis is falsified, he’ll resort to killing.
I think Harry’s mistake is that he has left himself no setting between, “no killing” and “all bets are off”.