unless the commonwealth THEN responded by putting a request for manslaughter or some lessor charge before the GJ after it declined to indict for murder. I don't think anything prevents them from doing that.
If as
@AugustWest explained above the GJ isn't given the leeway to decide to charge him with something lessor than the murder charge brought before them by the commonwealth, I assume the commonwealth has the option of revisiting GJ with a bid for a lessor charge.
Not that I think MS won't be indicted on the current charge, because I think it is hard to deny video evidence of the shooting and the fact that the judge was unarmed. I think it more likely he will be indicted and then when trial time comes defense will ask that the judge to instruct the jury on several other possible lessor charges they could find if they do not find MS guilty of murder. Then at trial argue the extreme emotional disturbance/manslaughter angle.