“It ought to, in principle, simply be necessary to inform the jury that they need to find that this homicide was NOT justifiable, beyond a reasonable doubt. That the task before them is NOT to find some better than likely excuse for it.”
That indeed does seem the way it ought to be, but I’m not sure. I thought I heard that in order to be immune on the basis of self-defense you have to argue that you were in reasonable fear of great bodily harm or death, and to do so you had to present evidence of some sort demonstrating that in your situation a reasonable person would be in fear.
I could see why you wouldn’t want juries to presume people were in fear for their lives, because justifiable homicide, though necessary, should be strictly reserved for cases that deserve it. Maybe practically speaking there is no way to distinguish between those who genuinely fear for their lives without having started fights in the first place from than to require the defense to assert positively that that was the case.
In order to have the best hope of reasonable doubt or better being found, it would make sense for a defense to offer such an excuse or justification, if consistent with evidence. But strictly speaking a defense isn’t absolutely forced to do it. It could, however foolishly, leave it totally up to the intuition of the jury.