Lets take another look at the issue

Lets take another look at the issue in the previous post.

We know that A killed B. A claims self defence. In another word A claims that B committed a guilt of attacking him in a way that justified a fatal response.

Now,if we want to apply the beyond reasonable doubt (or at least according to the way it seems to be taken) shouldn't we also apply it on A's claim of B's action that justified
a fatal response? If A's guilt in killing B can be proved only by proving beyond reasonable doubt that it was not because of self defence then it means that it is enough for just any reasonable doubt to prove that B is guilty of what A claims justified killing him which is lower than any known kind of standard of proof.