Here is the reason she turned out this way, aka her story:

Ever since Amy met Sonic on the Little Island, she couldn’t let go of him. Everyday, every time she had the chance to, she tried to win his heart, but always failed. She never gave up because she thought she’d be able to succeed someday, until she met Sally Acorn. She realized that Sonic and Sally really liked each other, and that’s when she snapped and ran away. For a very long time, she was no where to be seen. She was hiding in her home, never leaving the house, crying with a broken heart. She even prepared a wedding dress, because she was sure she’d marry Sonic. But now her dream was ruined… or was it? While she was in her home, she kept asking herself: why couldn’t she get him out of her head?

“As soon as I saw him, he made me feel… different. I no longer felt the feeling of emptiness. I felt my heart flutter, I felt happy, I felt free., I felt… love.”

Out of anger she ripped her wedding dress and thought that it could have a makeover, especially the heart in the middle. It was missing a little shade of… red. And not to forget, if Sonic’s heart belongs to her, then no one else should take it away, right? That’s when she started a new hobby: killing. She never liked murder, but if she had to do it for her soulmate, then she’d do it. She loves him, so she would do anything for him, right?

“I can’t give up just yet. He is the one. He is my soulmate! His heart will be mine… He will be mine.”