She felt insulted and disrespected for not receiving an invite (and Maryweather's "You weren't wanted" didn't help). So she punished the parents by cursing their child. Note that she didn't just kill Aurora on the spot, but made it so her death would hang over her and her family for 16, allowing relationships and bonds to form before having that stripped away.

It wasn't Aurora that Maleficent had a grudge against, she was just a means to punish the parents.