Is this just an argument to plead insanity? I wonder why this rarely works.

You were downvoted but you're right, live by the sword and take the risk to die by it. If you don't like the odds - and let's all hope the judicial system is efficient enough to make those odds really unfavourable - it is easy to avoid them by not committing capital crimes.

The sanest option is not to have capital crimes, that way nobody can commit them.