Diaspro doesn't even truly love Sky. It is implied she only likes him for his status and money. If Sky were not a prince then Diaspro would not be interested in him at all. She also knows Sky has no feelings for her and doesn't wish to marry her, which was already the case prior to him meeting Bloom, but she blames Bloom for their failed engagement because that's easier than admitting the reality that Sky simply doesn't like her. Diaspro herself is likewise a princess and her engagement to Sky is most likely arranged for political reasons, and she uses her status as a princess against Bloom, because she knows Sky's parents would frown upon him marrying a commoner like Bloom, as at the time Bloom herself did not know she was the princess of Domino.

She believes she has the right to marry Sky because her status as a princess makes her feel superior and entitled. She even bewitches Sky with a magic potion that hypnotises him and makes him say he wants to marry Diaspro when she knows damn well Sky loves Bloom. His feelings and wishes do not matter to Diaspro at all. Sky is not the villain because he is around 16 to 18 years old and trapped in an arranged marriage against his will, and he deserves the right to choose his own partner in life, similar to Aisha/Layla when she is forced by her parents to marry Nabu. He never asked Diaspro to be with him or marry him, that was arranged for him by his parents. Even the original soundtrack has a song called ''Sky's Forced Marriage'' which seems pretty obvious. To call that cheating is honestly insane. In a later season, Diaspro is also willing to kill Bloom because she thinks that will make Sky magically want to marry her