Who is following or monitoring the prosecutor?

Who controls the prosecutor?

Hello,

How is it ensured in Germany that public prosecutors actually bring charges against criminals?

As far as I know, only the public prosecutor's office can initiate criminal proceedings. I (as a private citizen) cannot simply file an indictment with the court and start such a process, even if I provide 100% evidence that the accused has committed a crime. As a citizen you have to kindly rely on prosecutors to do their job and actually bring criminals to justice.

But what if a public prosecutor doesn't want to charge a suspect at all? As a simple citizen, do you have any legal means to force a criminal trial against someone? How is the arbitrariness of the public prosecutors kept within limits in the German state - apparently they can decide at will (or according to instructions) which crimes are prosecuted and which not?

Let us imagine a theoretical example: a murder on the street in front of many witnesses, passers-by hold the murderer. But the prosecution refuses to bring charges "for lack of evidence". What can you do about it now? Charging the killer himself in court is not an option, and if you sue the prosecutor for "inaction" you will likely just get another rejection "for lack of evidence". What now?

The state could apparently tolerate any criminal offense here, in that the executive branch simply gives instructions to public prosecutors not to prosecute certain offenses. How is such a possibility of abuse of power prevented in the German constitutional state?