Yytz,
1. Rav Moshe did not share your rav’s concern (IM CM 2:68). BTW, a ben Noach can be executed (according to the Chelkat Yoav and Rav Aharon Soloveichik that is only a maximum penalty) for violating any of the sheva mitzvot. If a person goes down to the level of murder he has certainly already violated several of them.
2. The Torah system is only for a time when Am Yisrael are all tzaddikim and crime is very rare (Maharal, Chiddushei Aggadot Makkot 7a). In “normal” times it would result in hefkerut (Ran, Derasha 11).
3. Who says that there are no attorneys. The witnesses are also accusers and according to Prof. Nahum Sarna there was a public prosecutor in Tanachic times whose title was Satan (which can mean “accuser” or “enemy”). Chazal speak of kategors and senigors in Heaven so apparently it was in use in their time. They also say that anyone, including the judges, can speak for against the defendant (Sanhedrin 32a).
4. One can also say the opposite. Being that the prosecutor does not want a loss he will decline to prosecute, as is his right, where he does not think that there is a strong chance of obtaining a conviction.
5. The defense attorney’s job is also to see that justice is done and not rack up acquittals. However, he is just a macho as the prosecutor. In amany case he is even a former prosecutor who decided to make more money.