They could argue that if she came forward it would reduce American's faith in their government leading to instability or that it would provide an opportunity for our enemies to use the scandal to undermine national interests, or they could argue that the "vindictive ex" might expose secrets that she learned while in proximity to government officials, or even just lie and say she stole the secrets.

The president doesn't even have to murder her. The president can now disappear people in the middle of the night and ship them off to gitmo under the banner of "national security" and not tell anyone about it, and even if someone leaked that the cells in Guantanamo Bay were filling up even the supreme court wouldn't be allowed to see the evidence. Any trial at all would take place in secret military courts closed to the public.

This was the farthest thing from a moderate decision. It puts the president literally above the law. There is zero need for this kind of immunity when the actions a president takes while in office are legal, which is how we've gone nearly 250 years without ever once needing it.