When Medusa decided to not go back with Uriel and her friends she really didn't think things through. She did know that both Tina and Mark were heavenly rank hunters and it made sense for each party to be balanced because that would make it more difficult for cryptids to spot them. But she didn't count with Tina being Uriel's ex girlfriend.
Not just that, after Uriel accused Tina of being with him only to get good grades she felt the need to prove him wrong. The gossip of Tina being too close with Mark right after the breakup was already bad enough, but If word got out that she was a manipulative woman it would make her look bad. That was something Tina, as the heir to the Celes company, couldn't allow.
Unfortunately for Medusa, Tina had noticed how she acted towards Uriel and she quickly realized she had a crush on him. If Uriel moved on from Tina, she wouldn't have the chance to redeem herself on the eyes of the public opinion and she needed to do something about it.
