Why did Mary Bell kill 2 people?

Answer 1

In the movie Dead Silent, when she was alive in the 1940's a little boy called out at her ventriloquist show that she was a fraud. Later on the boy went missing and Mary was blamed for it. The towns people hunted her down, cut out her tongue, killed her, and buried her with all her dolls. She vowed to get revenge, thus she kills to exact said revenge.

Answer 2

Mary Bell killed because she said that she liked to kill young things because they couldn't fight back. She also had a history of sexual, and physical abuse by her mother.