The meaning is, slay every principal offender whom you meet with, without any indulgence or exception, though brother, or companion, or neighbour. There was no fear of killing the innocent in this case, because,
1. The people were generally guilty.
2. Moses had called to himself all that were on God's side, who thereby where separated from the guilty.
3. The innocent might easily be discerned from the transgressors, either by the personal knowledge which the Levites or others had of the most forward idolaters, or by their abiding in their tents as ashamed and grieving for their sin, whilst the transgressors were impudently walking about in the camp, as trusting to their numbers; or by the direction of God's providence, if not by some visible token.