כָּל־מַ֨כֵּה־נֶ֔פֶשׁ לְפִ֣י עֵדִ֔ים יִרְצַ֖ח אֶת־הָרֹצֵ֑חַ וְעֵ֣ד אֶחָ֔ד לֹא־יַעֲנֶ֥ה בְנֶ֖פֶשׁ לָמֽוּת׃
װער נאָר עס דערשלאָגט אַ נפֿש, זאָל מען הרגען דעם מערדער לױט דעם מױל פֿון עדות; און אײן עדות קען ניט עדות-זאָגן אױף אַ מענטשן, ער זאָל שטאַרבן.
Rashi
כל מכה נפש וגו׳ HE WHO KILLETH A PERSON, etc. — This means: He who proposes to kill him because he had killed a person,
Chizkuni
ועד אחד לא יענה בנפש, “a solitary witness must not testify against a person (in a capital case).” We might have thought that seeing that the penalty for capital crimes is so severe that the testimony of a single witness is relevant. The Torah tells us that it is not.
Siftei Chakhamim
The one who endeavors to kill him. Referring to the killer, because he had killed a person.
Rav Hirsch
צוטריטלעך נאָר פֿאַר פּרעמיום־מיטגלידערRashi
לפי עדים ירצח BY THE EVIDENCE OF WITNESSES ONE SHALL SLAY [THE MURDERER] — who testify that intentionally and after due warning he killed him.