The Torah's laws of homicide use masculine language: "If one strikes a man" (Exodus 21:12). The Mekhilta recognizes that this phrasing could be read as limiting the death penalty to specific combinations of killer and victim. The verse, combined with the Leviticus passage, explicitly covers four scenarios: a man who kills a man, a woman who kills a man, a man who kills a woman, and a man who kills a minor child.
But what about a woman who kills another woman? Or a woman who kills a minor? These combinations are not explicitly mentioned in either verse. The masculine language of "one who strikes a man" could be read as excluding cases where the victim is female or where both killer and victim are women.
The Mekhilta resolves this gap with a verse from Numbers: "The murderer shall be put to death" (Numbers 35:16-17). This verse uses the broadest possible language. It does not specify the gender of the murderer or the victim. It simply says "the murderer," without qualification. Anyone who commits murder, regardless of gender, is subject to the death penalty. And the victim's gender is equally irrelevant.
The Numbers verse "comes for this teaching," meaning its purpose in the Torah is specifically to cover the cases that Exodus and Leviticus left unstated. A woman who kills another woman is a murderer. A woman who kills a child is a murderer. The word "murderer" encompasses every possible combination of killer and victim, closing every gap that the more specific language of the earlier verses might have left open. The death penalty for homicide is universal, applying to every person who takes another person's life, without exception.