Alastor is making a strong case for being a Dexter type of character
A serial killer who went after other serial killers and used voodoo rituals so as to gain the power and take them out but it didn’t worked until he was dead hence why he had all that power, it just ended up being accumulated and then unleash.
It also makes sense how unrepentant he is about the stuff he did and how he doesn’t believe anyone can be redeemed after the fact. He most likely gave (very sadistic) choices to his victims on trying to be better but given his targets they of course wouldn’t. Alastor killed people and he most likely enjoyed it, doesn’t matter if they deserved it or not, a kill is a kill and he knows it.
So in a way, yeah he gets sadistic glee from desperation of people who most likely did horrendous things in life, but he also could have a desire to be proven wrong and still will get entertained because hey, the little naive darling actually pulled it off.