bad: the ordinary, all-purpose opposite of good, covering a corrupt character, a wrongful act, and plain harm alike
This is everyday Greek, not a specially religious term. Long before the New Testament it was simply the negative counterpart of "good": a shabby coat, an incompetent doctor, a dangerous animal. The New Testament writers picked it up exactly as they found it, and it does most of the work that plain English "bad" still does.
Because the word itself is so general, the setting decides which shade is in play:
- a person whose character is corrupt, like the servant who beats his fellow servants
- the wrongful act itself, the crime a court would have to name and punish
- the injury one person inflicts on another, and the plain damage a snakebite or a sore does
- the bad things that simply befall someone, like the miseries Lazarus lived with
None of these is the word's real meaning with the others borrowed. All four are ordinary Greek.
Its commonest use in the New Testament turns the adjective into a noun, "the bad," meaning the wrong itself. Paul reaches for it that way more than any other writer. In the clustered command not to repay anyone "evil for evil" (Romans 12:17; 1 Thessalonians 5:15; 1 Peter 3:9) the one word covers both sides at once: the wrong done to you, and the wrong you are tempted to hand back. That single word doing double duty is the point of the saying.
A well-known older claim is that this word names a passive kind of badness, a simple lack of the qualities something ought to have, while the stronger near-synonym (πονηρός) names badness actively at work on other people. The text itself makes that hard to hold. Mark 7:21 calls the thoughts that come out of the heart "evil" with this word, the parallel in Matthew 15:19 uses the other one, and Revelation 16:2 piles both onto the same sore.