The Mekhilta presents a sophisticated chain of legal reasoning about which commandments can override which other commandments. The question at stake is whether the obligation to bury an unattended corpse (a meth mitzvah, a body with no relatives to claim it) overrides the Sabbath.
The argument proceeds through a series of careful comparisons. The sacrificial service in the Temple is overridden by the burial of a meth mitzvah. If a priest encounters an unattended corpse on his way to perform the Temple service, he must stop and bury the dead. The dignity of the dead takes precedence over the Temple ritual.
The sacrificial service is also overridden by murder, meaning that the obligation to execute a convicted murderer takes precedence over the Temple service. If the execution and the sacrifice conflict, the execution wins.
Now the Mekhilta considers the Sabbath. One might argue that since the burial of a meth mitzvah overrides the Temple service, and the Temple service is a serious obligation, then burial should also override the Sabbath. But the Mekhilta blocks this inference. The Sabbath has a different legal profile than the Temple service. The Sabbath is not overridden by murder. A convicted murderer is not executed on the Sabbath. Since the Sabbath resists even the powerful obligation of executing a murderer, it cannot be overridden by the lesser obligation of burial.
The reasoning demonstrates that commandments do not exist on a simple linear hierarchy. The fact that obligation A overrides obligation B, and obligation B overrides obligation C, does not automatically mean that A overrides C. Each relationship must be established independently. The Sabbath occupies a uniquely protected position in the hierarchy of Jewish law, resistant to overrides that other sacred obligations must yield to.