The Torah uses masculine language when describing the crime of kidnapping. (Deuteronomy 24:7) says "if a man be found to have stolen," and (Exodus 21:16) says "one who steals a man." The Mekhilta systematically works through the gender combinations to determine who is liable and who is covered.
From the Deuteronomy verse alone, the law covers a man who steals another man. Combined with the broader language of Exodus, the coverage expands. A man or a woman who steals a man is liable. A man who steals a woman is liable. A man who steals a minor child is liable. These combinations are derivable from reading the two verses together.
But certain combinations remain unaddressed. What about a woman who kidnaps another woman? Or a woman who kidnaps a minor? The masculine language of both Exodus and Deuteronomy does not explicitly cover these cases. If the law is limited to its literal gendered language, a female kidnapper who targets women or children might escape capital punishment.
The Mekhilta closes this gap with a third verse. (Deuteronomy 24:7) concludes: "Then that thief shall die." The word "thief" (ganav) in Hebrew is grammatically inclusive. It does not specify male or female. Any person who commits the theft of a human being, regardless of the thief's gender and regardless of the victim's gender or age, falls under the death penalty.
This verse "comes for this teaching," meaning its specific purpose in the Torah is to ensure that no combination of kidnapper and victim escapes the law. The final phrase sweeps away every possible exception that the gendered language of the earlier verses might have created. Kidnapping is a capital crime for everyone who commits it, against everyone it is committed against.