No way they were going to prove he didn't turn back beyond a reasonable doubt.
I wasn't aware "beyond a reasonable doubt" was a criterion for a grand jury to determine probable cause.
Correct, but I imagine the prosecutor knew from the start that proving criminal culpability "beyond a reasonably doubt" would be exceedingly unlikely based on the evidence. He punted it to the grand jury to satiate the baying mob. As an attorney and prosecutor he is held to certain ethical standards and himself could face sanctions (including disbarment) for prosecutorial misconduct if he filed criminal charges without a reasonable investigation and foundation. It seems that some wish he had done that. By providing all the available evidence to the grand jury, he essentially showed that his initial decision not to press forward was right.