Why did the snake demon chase the soul down?

Hello, I want to ask what is the snake demon chasing the soul? This is a movie.

The Snake Chasing the Soul is a horror film directed by Gam Fung Lam and starring Wu Jiali. Lin Sihui was actually reincarnated as a snake lady, and her father died unexpectedly before she was born. After she was born, she cried for several days, so she asked the old mage to tell her fortune, pointing out that she was reborn as a snake girl in her previous life and would be a coward in the future. Unfortunately, she could not raise shit, so she was given to the snake prince to live a peaceful life. The film is 100 minutes long.