What changes the balance between good and evil? Your actions.

To see evil and do nothing is evil.

Evil and good are Siamese twins, bound at the hip.

All good and all evil comes from within us, as does our projection of it onto people, places, and things outside us.

Good and evil are so completely entwined any action can be good or evil.

You can't correct the past evils, because you can't change the past. What you can do is learn from the past and use this knowledge to change the present, thereby the future.