Alastor is not an introvert
He may appreciate his alone time. He prefers keeping his secrets to himself
Other people are sometimes just toys for his own amusement
And yet, he is not an introvert
We know he regularly went to speakeasies. He even kept in connect with someone long enough for that person to still consider him a friend a century later (Mimzy).
He's been shown attending parties
He likes talking to people. Yes, even if it is to mess with them. But he still does
He didn't have to stay for the "We may die so let's live!" celebration in the season 1 finale. And yet, he did. And he said "It's been a surprising thrill to witness these wayward souls find connection. Almost makes one sentimental."
That does not sound like someone who prefers to be alone. That sounds like someone who has made a lot of social connections or at least appreciates it
If Alastor had everything he ever wanted, all the power in the universe, BUT he'd be the only person in it?
Because he relies on other people for fun
And Alastor craves that, craves doing whatever he wants to do, 'continuing his fun', without any consequences
Jazz, his favorite music genre, has call-and-response as a key component, where one instrument will join in by answering to the sound of another, building upon it
He feels most satisfied when he is the strongest and smartest person in the room. But being the only person in the whole building kind of nullifies that
He can't analyze people and use their weaknesses against them or rip them apart piece by piece when there are no other people
Alastor is a social person. Not an empathetic one, but he enjoys being around others, that's where he gets his energy from