(This only scales to the point where you value integrity however: you may be able to live with yourself better after finding you're self deluding than after murdering 15 people to prove a point)

This is precisely my reasoning too. It doesn't seem at all sensible to me that the principle of "acting as one would formerly have liked to have precommitted to acting" should have unbounded utility.

ETA: When you say:

Making credible threats requires us to back up what we say, even to someone who we will never encounter again afterwards, so similar situations (without the absolute predictive ability) are quite common in life. I know in the past I have acted perversely against my own self-interest to satisfy a past decision / issued threat.

Now this seems a very good point to me indeed. If we have evolved machinery present in our brains that predictably and unavoidably makes us feel good about following through on a threat and bad about not doing so - and I think that we do have that machinery - then this comes close to resolving the problem. But the point about such a mechanism is that it is tuned to have a limited effect - an effect that I am pretty sure would be insufficient to cause me to murder 15 people in the vast majority of circumstances.