Already considered "smitten"
In the begining of this weeks Parsha, it is written: "And the name of the Israelite man who was smitten, who was smitten with the Midyanite, was Zimri Ben Salu the head of the tribe of Shimon". The holy Ohr Hachaim Hakadosh, whose Yartzeit is today, asks why the Torah wrote twice the words "who was smitten". The Ohr Hachaim answers that the Torah is telling us that Pinchas didn't kill a living "Yisrael" (Israelite) but rather he killed a smitten "Yisrael" - a man, who through his sinning with the Midyanite, was already considered "smitten", since in doing this sin he smote the "Yisrael" part of his soul. That's why it says "the name of the Ish Yisrael (the Israelite man) who was smitten" - as if to say, his "Yisrael" was smitten already, even before he was killed by Pinchas.
In another explanation the Ohr Hachaim writes that it could be that through his death he atoned for his sin, and therefore the Torah calls him an "Ish Yisrael" as if to say that, even though he sinned, he was still considered a Yisrael.
It is also interesting to note (as pointed out by the Sefer Hamusar) that we see from this week's Parsha that sinning with a gentile woman is considered so severe a sin that it is the only sin in the entire Torah that it is permitted to kill the offender without witnesses and without warning (Hasra'ah), as Chaza"l say "Kanayim Pogim Bo" - "Zealots are allowed to kill him".