shmoel –
Or… you witness an actual maisa znus (eishes ish).
Not only is that incorrect, but you would be liable to the death penalty if you were to kill someone involved in znus with an eishes ish, if they are consenting adults. Only if it is rape does the perpetrator have the halacha of a rodef.
The Pinchas halacha is something else entirely. It only applies to a Jewish man who is being mezaneh with a non Jewish woman in public (at least ten people watching). And even in this case, if the Zimri turns around and kills Pinchas he is justified according to halacha and will not be persecuted. The only thing Pinchas has is the right not to be persecuted if he kills them in his zeal for Hashem’s honor.
Zimri: the guy being mezaneh.
Pinchas: The zealot.