The only thing that doesn't make sense about the curse is that they didn't tell him what the trigger was. Otherwise - it makes perfect sense. If Angel can have his soul and there isn't the perfect happiness clause then he can choose to move on from the torment. He's going to live forever - there's only so long he can beat himself up about terrible things he did hundreds of years ago whilst he was a completely different person. Without the trigger, Angel can live a normal, happy and eternal life. That isn't a punishment.
The point is he is forced to feel that burden of what he did, forevermore, for fear of reverting back to that creature. He can't move on and get over it - because of the trigger - so he has to suffer. He has to make sure he is always suffering. He can never know a moment's peace and he is responsible for ensuring he never gets that peace. That is torment - for all eternity.
However ... not telling him the clause makes zero sense, as, as we see, in his ignorance he just kinda blunders into it anyway. He isn't guarding against his own happiness, because he doesn't know he has to, and so isn't being tormented the way the Romany people want him to be. The curse makes much more sense when he gets his soul back as - now he knows about the curse - he is forced to police his own happiness and make sure he is always a little bit miserable. Which, I suppose, is what the Kalderash people wanted ... but they really ought to have told him that.
I came to the conclusion years ago that the original Romani set the curse up like this deliberately. The real agony is that he works his way back to happiness, loses it in the worst surprise ever, becomes a murderous asshole again, and then if someone puts it back on--then he knows he can literally never be happy again. Vengeance is a living thing. It's the ultimate torture.
The flaw in this plan is that it doesn't sound like the current generation of Kalderash knew about the latter part of this so much? Maybe it was forgotten with time or something, though it seems like these folks wouldn't be the sorts to forget.
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