As someone who had to sort of rely a bit on well known people to have some good models and examples in life due to lack of close relative and friends(apart from my mom and one grandfather who died pretty early on as I hit my teens), I understand that investment in certain celebrities.

In a way I think you can get a good reading of a person's personality from afar depending on how available information is and opportunities to watch them(live or in video) interacting with other and behaving. But of course, as with everyone who isn't a very close friend or family, there's always the possibility there's facets to them you aren't aware of so there's room for disappointment.

From what you describe it's not so clear cut he was douchey because that's how he is or it was an unfortunate moment when he was unintentionally rude because he was tired or upset for some reason.

I do think that artists should be civil and fairly accommodating with fans who are polite and decent when approaching them(as u surely were), because ultimately it's not that they owe you their time but that their whole job, their livelihood is dependent on people responding positively to their work and investing time and money and feeling in it. No actor, artist or author of any kind would be anything without fans and admirers of their work and person and there's a lot of fans who spend more money and time than they can afford just to support an artist and see them perform live. Performing and creating and being in the spotlight is hard but they get paid millions for it while fans often sacrifice time and money they might not have and the payback is simply a live, first hand experience.

So yes, artists like that should be thoughtful and appreciative of their fans and try to be friendly with friendly fans. But ofc celebrities are humans too and any human can and will have bad days when they're just not in the right place mentally and it may come off as rude.

It's hard but try not to take it personally cause it's either he was just having a bad day or he's an asshole in general. Maybe try to see what other fans have to say after encounters with him

My favorite actors are both more introverted and quite uncomfortable with anything fame related. One of them, Hugh Laurie, I've seen in concert a few times and the first time I got the impression he was a bit uncaring cause he didn't stop to give autographs or anything after the show. The other time I saw him though, he not only gave autographs but took his time with absolutely everyone waiting, took pictures and chatted and he was absolutely lovely. That first time it was his first true concert and he later said he was nervous and exhausted. With introverted or weird people it can happen that they seem rude when they're not really.
The other actor I like is not that well known, mostly a theatre actor, introverted and quite awkward about meeting random people like this and very uncomfortable. He refuses most picture requests and I thought that was iffy but I had the chance to meet him in a slightly different context and he is one of the kindest, most wonderful people you can meet but he can easily come across as distant and closed off in the wrong setting.

Try not to take these things personally, although it is understandable that u do.