A Midianitish Woman Is Brought Into the Camp by a Simeonite Chieftain (Numbers 25:6).
‘And, behold, one of the children of Israel came and brought to his brethren a Midianitish woman in the sight of Moses, and in the sight of all the congregation of the children of Israel, while they were weeping at the door of the tent of meeting.'
But even while the judges were meeting, and there was weeping at the door of the Tent of meeting, because of the sin of Israel and presumably because of the plague which had now broken out, ‘one of the children of Israel' (a Simeonite chieftain - see Numbers 25:14) boldly and blatantly brought into the camp a Midianitish woman in the sight of Moses and all who were gathered before Yahweh. He appears to have had no shame in the matter. He presented her to his brethren before taking her to his ‘pavilion' or inner portion of the tent. His open and brash involvement with the Midianite women was made very clear. It was high handed sin.
Prior to this it would appear that all the ‘sinning' occurred outside the camp. So this was an increase in offence by the introduction of idolatrous behaviour into the holy camp of Yahweh. That was what justified Phinehas' instant action.