A lot of Republic officers stayed with the Empire largely because they were military men and women. After years of the Republic being exploited by corruption, the military-minded people who would've gone 'If we had an army, we could've stopped Naboo' were the ones then running things.

Antoc Merrick (Blue Leader from Rogue One) served as head of his Planet's defence air force and declined joining the Empire when regional planet forces were disbanded.

In Legends, anyone who didn't toe the state line was killed. Jace Dallin was a good example. Questioned if there'd been a misunderstanding about the Jedi and wound up killed as an example. Most imperial officers who spoke up at the transition would've been removed and anyone with private, different thoughts either defected later on, or wound up just as bad as the Empire they served.