Or HaChaim on Numbers 31:8/אור החיים על במדבר ל״א:ח׳

1 ואת מלכי..חמשת מלכי מדין, "and the kings of Midian….-the five kings of Midian." The reason that the Torah spelled out their number (i.e. 5), something we had already counted ourselves, was to tell us that they were all killed next to one another so that all the soldiers could count the bodies of these five kings whom they had slain. The reason why the Torah repeats that they were kings may have been to tell us that they were killed by soldiers who knew full well that these five men were kings. They were not the victims of random killings.

2הרגו בחרב, they killed by the sword. The Torah had to write the word הרגו, "they killed" once more when describing the manner of Bileam's death though we gleaned his information already from the context (end of verse 7). The reason is best understood when we read Sifri where it is stated that Bileam had gone there to collect his fee for the diabolical advice he had given Balak with such stunning results. According to Sifri, at that moment Bileam was on the way to advise the Israelites not to make war against Midian saying to them that if they had been unable to overcome Midian when the were able to field 600.000 fighting men, now that they numbered fewer they would certainly not be able to do so. According to the Sifri that the Israelites killed Bileam as a response to his effrontery, he was killed before the battle against the Midianites had taken place. This was all part of G'd's clever plot. He wanted to demonstrate that Bileam did not die because he was in the wrong place at the wrong time, but that He, G'd, had arranged things so that the Israelites would have an excuse to single him out for killing. Had the Torah not repeated the word הרגו unnecessarily, we would not have been able to deduce this from the text. The emphasis on the word בחרב, "by the sword," is to show that all his sorcery did not help Bileam and instead of their "killing" only his alter ego they killed his body.