Phinehas and Corpse Contamination

Phinehas was a son of Aaron, the High Priest.  In Numbers 25:7-8, Phinehas kills with a javelin an Israelite man who brazenly brought a Midianite woman into the presence of Moses and the congregation of Israel, along with the Midianite woman.  This was when God was particularly upset with Israel because Israelite men were getting together with Moabite women and were worshiping Moabite gods.  Later, in Numbers 31, Israel is battling against Midian under God's command to punish Midian because the Midianites tempted the Israelites to betray the LORD.  And who went out with Israel into battle?  According to Numbers 31:6, Phinehas did.

But I have a question.  In Leviticus 21, there are strict rules against the sons of Aaron defiling themselves with a corpse.  Leviticus 21:1-4 state (in the King James Version): "And the LORD said unto Moses, Speak unto the priests the sons of Aaron, and say unto them, There shall none be defiled for the dead among his people:  But for his kin, that is near unto him, [that is], for his mother, and for his father, and for his son, and for his daughter, and for his brother, And for his sister a virgin, that is nigh unto him, which hath had no husband; for her may he be defiled.  [But] he shall not defile himself, [being] a chief man among his people, to profane himself."  Did Phinehas defile himself with corpses by killing the Israelite man and the Midianite woman, and by going out to battle against Midian?  Would that have gone against the rules in Leviticus 21 against the sons of Aaron defiling themselves with a corpse (with a few exceptions)?

Perhaps one could argue that Phinehas killed the Israelite man and the Midianite woman but did not touch any corpses, and thus he escaped corpse contamination that way.  But, in Numbers 31:19, both the Israelites who killed Midianites in battle as well as the Israelites who touched a corpse had to purify themselves on the third and seventh days, which, incidentally, are the same days that people purify themselves of corpse contamination (Numbers 19:12, 19).  That tells me that one becomes contaminated by a corpse by killing someone.  

I have not yet found any Jewish explanations of this dilemma.  What I have read in rabbinic literature is that Phinehas in Numbers 31 was continuing the fight against Midian that he began in Numbers 25.  But I wouldn't be surprised if a Jewish source somewhere addressed it.  A while back, I wrote a post in which I asked how Samson, a Nazirite, could kill Philistines in battle, when Numbers 6:6 prohibits Nazarites from going near a corpse.  A commenter responded that, according to a Jewish tradition, Samson was a special type of Nazarite for whom that rule did not apply, and I later read that same point elsewhere.  Was there was a halakic way to justify what Phinehas did, within rabbinic (or later Jewish) literature?