What is the primary goal when contemplating planetary defense against a massive comet?

Deflection, rather than destruction.

In scenarios where advanced warning is available for a massive celestial object approaching Earth, the strategic objective shifts entirely toward planetary defense rather than focusing on post-impact survival blueprints. The primary methodology endorsed is deflection—the act of gently nudging the object off its collision course well in advance. Destroying a large body is generally avoided because breaking it into numerous smaller pieces might inadvertently transform one singular catastrophe into several, potentially guaranteeing widespread devastation across multiple zones instead of concentrating the threat in one area.