Kakos (G2556) is one of two main Greek words for evil (the other being poneros). While poneros emphasizes the active, malicious nature of evil, kakos focuses on evil as the absence or corruption of what is good β badness in character and quality. Paul uses it extensively in his ethical teaching.
Romans 12:17-21 builds an entire ethics around kakos: 'Do not repay anyone evil [kakos] for evil [kakos]... Do not be overcome by evil, but overcome evil with good.' This is the Christian revolutionary ethic β not the absence of kakos but its active defeat through goodness. 1 Peter 3:9 echoes it. The battlefield against kakos is not political or military β it is moral and spiritual.