Final answer:
The correct word to fill in the blank is 'expiate', which is about making amends for wrongs by atoning for guilt or sin.
Step-by-step explanation:
After he had seen the error of his ways, the villain attempted to expiate the dark deeds of his past by acts of kindness and mercy. The word 'expiate' means to atone for guilt or sin. This concept is about making amends, often through some form of punishment or compensation, for wrongs one has done. It aligns with the theme of seeking redemption through positive actions after acknowledging one's misdeeds.