The Law of Conservation of Good and Evil
Physics has physical laws such as Law of Conservation of Energy.
I propose the Law of Conservation of Good and Evil. In systems/people/institutions to which it applies, the LCE says that over the long run, the amount of good and evil evens out. It’s an empirical question whether the law applies to specific systems on earth. My guess is that it applies to most religions.
On the one hand, religions teach people to care for others and help the poor and the sick. Abolitionism and the Civil Rights Movement were largely led by religious folk. Many devout religious people are loving and humble. Often they donate money, both to help their religious organization and to help disadvantaged people.
On the other hand, religious intolerance and superstition often lead followers to support demagogic and reactionary politicians who cause wars, discrimination, corruption, inequality, and waste. The ascendancy of the Republican Party in the late 20th century was largely due to support by the religious right. Republican policies resulted in much death, destruction, and injustice: unnecessary, corrupt wars that killed and maimed millions and wasted trillions of dollars; reckless deregulation that harms our health, crashed the economy, and damages our economic well-being; violation of rights of women and gays; and increasing concentration of wealth and power.
Also, while many religious people are loving and humble, many religious people are self-righteous, deluded, condescending, intolerant, and vindictive (especially if you question the veracity of their religious dogmas).
The LCE may be a consequence of the Second Law of Thermodynamics, which states that the entropy (disorder) in an isolated system always increases. Both good and evil would seem to require effort and order: it takes work to help someone or to harm someone.
The LCE would bode poorly for Martin Luther King’s hope that the arc of history bends towards justice.
There is some evidence that the arc of history does bend towards justice: there is less war, violence, and exploitation now compared to the past. Slavery is outlawed in most places. Women are gaining rights. Education and government (yes, governments!) have brought peace and civility for most people. It’s unclear whether such progress will continue.