Mermaid Gothic (2014)

Miletis, the country at the bottom of the ocean in which the merfolk live.
It was once a beautiful blue, but now, it is as black as a wicked darkness.

The protagonist is a mermaid princess.
When she was a child, she committed the crime of saving a human's life. As a result, she is incarcerated in a tower.

The months turned to ten years. It was then that her father, the king of the merfolk, issued an order in exchange for her to be free.

"Kill Basil Adelworth, the prince of Armelia, a human nation."

To carry out the deed in order to atone for her crime and be free...
Or to refuse to dirty her hands and resign herself to live the rest of her life within the tower...

The mermaid leaves the ocean to steal the life of the one she had saved previously.