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.
How do you know so much about the social dynamics of medieval fairies
Another interesting thing about fairies is that there’s not all that much distinction between the ones that are good and the ones that are evil. Sometimes they give you blessings that are obviously awesome, and sometimes they’re like “I bless you with the gift of being fucking ugly. You will be an ugly baby, and an even uglier kid, and you will be the ugliest lady to ever live.” and it’s meant as a blessing, because being ugly means you’ll eventually end up with a husband who actually loves you. Getting a fairy blessing is a total crap shoot most of the time. Ella Enchanted is a great modern take on the double-edged sword that fairy blessings are.