What is the ruling on a Muslim's fasting while he is an oppressor?

The purpose of fasting is to achieve taqwa (piety/God-consciousness) of Allah Almighty. Every sin committed by the fasting person, such as backbiting, lying, slandering, and injustice, diminishes the reward of their fast, to the extent that the reward may be entirely lost, as the Prophet (peace and blessings of Allah be upon him) said: "Many a fasting person gets nothing from his fast except hunger." However, the validity of the fast is not nullified by committing a sin; rather, its reward is diminished thereby.