In medieval culture, an event like a royal christening is not a private party; it’s the public social event of the year. To not invite any person of rank to such an event is a deadly insult.
Maleficent is certainly someone you wouldn’t want at a party, but she’s also someone powerful enough that only a fool would ever dare treat her with such blatant disrespect. The only way the King and Queen could possibly have gotten away with not inviting Maleficent was to not invite any of the fairies at all; inviting the other fairies and excluding her is explicitly taking sides in the conflict between the fairy factions.
Which means they made themselves her sworn enemies, and she responded by treating them as such from then on. If you actually get into analyzing the social dynamics of the scene, it’s very clear that Maleficent was willing to show mercy at first by giving the King and Queen a chance to apologize for their disrespect to her. She doesn’t curse Aurora until after she gives them that chance and they throw it back in her face with further disrespect.
And yeah, if the King and Queen had done the properly respectful thing and invited her, Maleficent would have given Aurora a scary awesome present. Moreover so would the other fairies, because at that point both sides would be using it as an opportunity to show off and one-up each other. What they gave her before Maleficent showed up was basically just trivial party favors by fairy standards.
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To be fair to King Stefan and the Queen, they might have apologized. For that matter, it might really have been an oversight - I’ve read versions of the Sleeping Beauty story where it was a complete accident that the bad fairy wasn’t invited. Usually these are the versions where there are dozens of fairies, not just four. In those instances, the King apologizes and does everything he can to make amends for the slight, but the bad fairy prefers to hold a grudge anyway.
Here, instead, Merryweather had to butt in and mouth off to Maleficent, leading to the curse. If she’d kept her mouth shut for five minutes, the actual hosts of the party - who were the only ones who had the right to say who was and was not wanted there - might have had the chance to smooth everything over and appease Maleficent.
(Of course, we wouldn’t have much of a movie if that happened, but it’s still a really egregious use of the Idiot Ball.)