"Except [by] one who has been wronged" (illā man ẓulima): This is an exception to the "speaking evil" (jahr bi-l-sūʾ) which Allah does not like. It refers to the oppressed person speaking out, which means invoking [Allah] against the oppressor and mentioning the evil that is in him.
It is also said: It means to initiate the insult, so he responds to the one who insulted him.
[Reference:] "And whoever defends himself after having been wronged" (Ash-Shūrā: 41).
It is also said: A man was a guest of some people, but they did not feed him. He began to complain, and was rebuked for his complaining, so this [verse] was revealed.
It is also recited as illā man ẓalama (active voice), implying a disjunctive exception (istiṯnāʾ munqaṭiʿ), meaning: "But the oppressor is committing what Allah does not like by speaking evil."
It is also permissible for man ẓulima to be in the nominative case (marfūʿ), as if it were said in the sense of: "No one came to me except Zayd," meaning "None came to me except Zayd."
An example of this is: "None in the heavens and the earth knows the unseen except Allah" (An-Naml: 65).
Encouragement to Pardon
Then, He encourages pardoning, such that no one should speak evil against another, even if it is by way of defending oneself after having been wronged.
Although He permitted speaking evil and made it permissible, He encouraged what is more beloved to Him, more excellent in His sight, and more indicative of nobility, humility, and servitude.
He mentioned "revealing good or concealing it" to draw attention to [the virtue of] pardoning. Then, He linked [pardoning] to them, regarding it as significant and drawing attention to its status, showing that it holds a central place in the category of good deeds.
The evidence that pardoning is the intended purpose behind mentioning the revealing and concealing of good is His saying: "For indeed, Allah is ever Pardoning, Competent."
Meaning: He pardons the offenders despite His competence to exact retribution, so you must follow the way of Allah.