dear Diogenesister...
Some life changing events happened between moses taking a midianite wife and the other guy taking a wife. Moses wife recognized the need for circumcision as a special sign of covenant which indicates that she had some desire to please God.
There is no indication in the text that the later midianite wife (or her israelite husband) had any desire to please God. Reading the progression of events suggests the israelite groom was guilty of the very things the gathered elders were lamenting at the moment, ie. being led astray by pagan culture, worshipping idols etc.
The way I see it, Phinehas was praised and rewarded based on his zeal for God, His commands and His righteous honor, not for killing. Likewise, in our justice system people don't praise an executioner for executing, the praise is in the fact justice is served.
michelle