What was the primary driver necessitating the intentional destruction of the Galileo probe?

An overwhelming commitment to Planetary Protection regarding Europa.

The overriding reason for the controlled impact was NASA's Planetary Protection policy. This policy mandates that any spacecraft interacting with environments deemed potentially habitable, such as Europa due to its suspected subsurface liquid water ocean, must be disposed of in a manner that guarantees no possibility of future, accidental contamination. Even with advanced sterilization techniques available in the 1980s and 1990s, there remained a non-zero risk that terrestrial microbes clinging to the probe could survive a long, uncontrolled coasting orbit and eventually impact Europa. By driving the craft directly into Jupiter’s crushing atmosphere, engineers ensured complete destruction via extreme heat and pressure, effectively reducing the statistical likelihood of contamination to zero, which was deemed the only responsible ethical path forward given the potential scientific value of discovering life on Europa.