Beyond reasonable doubt is the basic principle of our criminal justice system.
Now in most states there is "affirmative defense" where the concept is you "admit" committing an act that is usually a crime (like killing someone) but claim it is justified based on XYZ (usually self-defense or defense of others, or a situation where a reasonable person would have felt seriously threatened). In many states that does change the standard of evidence somewhat... how much depends on the jurisdiction. Anti-SD states try to come as close to making you "prove it was SD" as they can without upsetting SCOTUS; some Pro-SD states only require that there is no reasonable proof that it was NOT.
So it could possibly vary in that sense.