For one thing Greg and Amethyst have been shown to be close in the past. Amethyst still likes Greg, and she can do something that no human woman can do: become his lost love. One day they're watching Lil' Butler, and one thing leads to another and Amethyst transforms so Greg can spend some time with “Rose.”

Why did Amethyst shapeshift into Rose?

This made Amethyst really upset since she was really close to Rose, most likely something to do with Kindergarten. So Amethyst probably shapeshifted into Rose in an attempt sabotage Greg and Rose's relationship, or to do something else that made a giant rift between them.

What was the relationship between Amethyst and Greg?

Her actions towards him were very uneeded because Greg loved Rose very much, and she was just playing with it. She thought that Greg was selfish, only thinking about himself and not others. She thought that he not only took Rose away from them but also their friendship they used to share.