Very much this, but I'd also argue that whilst all the above is correct, I think the way he behaved at Chelsea about his time at Brighton ("could've stayed at Brighton and had an easy life"/"I did a good job and left Brighton in a good place" etc.) was as damaging as leaving.
At the time, I thought, incorrectly, that we were a big project for him. I know that he turned down a number of very good approaches from big sides before deciding Chelsea was the right fit for his future and I, perhaps foolishly, was convinced that he'd be with us until Southgate stepped down as England manager. It was doubly disappointing to me that the reward for our (and the club's) patience through those pretty stinking periods that led to those incredible, final 12 games under Potter was to watch him go to Chelsea, where he was always going to fail. That we ended up being painted by him as a stepping stone towards bigger things will always sting a little, even if in hindsight, it is an obvious truth that it would always be that way.
I think you can call him a top ten Albion manager, but still feel quite bitter towards him at the same time. He did a very good job in the time he was with us, but the club and fans also created a patient platform for him to succeed that very few other clubs would have. I'm convinced had he not had the remarkable turnaround from Arsenal away at the end of 21/22 and instead got the results we were all expecting to see he may have been out of a job that summer anyway (1 home win in 8 months, 1 goal in 7 games leading up to that). Roberto came in and proved scoring goals and dreaming big was in fact, possible, and delivered something far more memorable than a 9th place finish.
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