1. What “murderer without witnesses” case are you referring to?
If there’s no witnesses, why are you assuming the accused is even guilty of murder altogether?
2. Who do you think enforced non-Jews living in Eretz Yisroel during the times of the Sanhedrin with complying with the law (the 7 obligations are part their laws)?
If a non-Jew worshipped idolatry in Jerusalem, do you imagine that the Sanhedrin (when they were in FULL control with no outside power [i.e. the Romans]) would turn a blind cheek?