“Diana should be executed and Charles is off free…”
No matter what Charles did, the legitimacy of William would not be affected.
Legitimacy of a future Head of State is a big issue.
Here we have the whole “birther” movement, and Obama had to release his full birth certificate to try to quell them.
Our own King David had to deal with legitimacy issues because of Ruth and Moab. (Moavi v’lo moavit)