Why did Moshe declare "Mi laHashem eilei" (22:26) before charging the Bnei Levi with the task of killing their bretheren (ish b'achiv) who worshipped the eigel? This could not be a clarion call for all those who were not idolators to gather because the vast majority of the Jewish people took no part in the eigel worship!
The Netziv answers that the task of putting down the uprising of eigel worshippers was dangerous -- who says that these people who submit easily to the sword and not fight back? We know the rule of thumb (Pesachim 7) is that one is not obligated to do in a mitzvah where there is a high risk of danger. Therefore, there was no explicit command from Hashem to kill the crowd of idolators and no obligation to do so. However, writes the Netziv, such calculations of risk / benefit apply only to regular people intent on performing the mitzvah to accrue reward. If a person is overcome with a love of G-d and performs a mitzvah motivated only by that love, no amount of danger will stand in the way -- such shluchei mitzvah ainan nizokin no matter what the degree of danger present. It was these people who Moshe called out to gather to him to perform the task of killing.
The Netziv also notes that the killing was done "ish b'achiv", to bretheren, to people who the Levi'im recognized and were related to. There is always a danger when imposing a punishment that a little bit of personal animosity may creep in, maybe a little bit of joy at another's downfall. Not only did Moshe seek out only those motivated by love of G-d, but he told the Levi'im to stike only at those who they otherwise felt brotherly love for so that the punishment was delivered only with the purest intentions.