If the ordinary magisterium of the bishops teaches "Thou shalt not kill", it will be understood as "thou shalt not murder" if that is what the current magisterial voices say it means. If the current magisterial authority says it means "No capital punishment" (among other things) then that is what the perennial teaching means. These things do not come down to us without a cloud of witness surrounding them.

There are balances around this, but on these subjects - correcting the pope, communion for the remarried - the perennial magisterium has said whatever the Pope interprets it as saying. Establishing a different meaning is very difficult, almost impossible.