Why did Aegon III allow Aegon II’s decrees re; his mother Rhaenyra, to stand? You’d think one of his 1st acts upon reaching his majority would be to have the record refer to Aegon II as the usurping kinslayer he was.

A couple things:

1. Aegon III took over in the aftermath of a very ugly and protracted civil war, and a good bit of his reign was spent trying to make sure it didn’t start up again. Hence, marrying Aegon II’s daughter to try to unite those dynastic claims. Retroactively declaring Aegon II’s decrees void stirs up a lot of trouble, both by offending Aegon II’s partisans and by pissing off people who’ve relied upon royal decrees and legal judgments that are suddenly called into question. 

2. Stare decisis is really important in any system that relies on precedent. If Aegon III can undo the decrees of Aegon II, he’s got to worry that some king who comes after him undoes all of his decrees.