shin already killed the one who killed ouki ( hoken) what's left for shin is defeating rbk in a battle.
as we know from current chapters
rbk is not even close to shin strength.
so shin killing rbk become meh after recent chapters.
he needs to defeat him this what will affect rbk more.
but since shin is just a muscle without brain 🧠
it means kryoten( hsu) who gonna defeat rbk not shin
.
....
Also by that time everyone would have raped rbk once at least once
ousen and now kanki maybe next tou or mobu
so shin defeating him later is not really an achievement
....