Why is the killer not told, "Repent and conceal your sin," as the fornicator is told, even though both violate the rights of others?

Murder involves three rights: the right of Allah, which is waived by repentance; the right of the guardians (victims' families), which is waived by the killer surrendering himself to them, or by retribution (qisas), or by their forgiveness; and the right of the murdered person, which it is hoped Allah will bear on behalf of His repenting servant.

As for adultery with a married woman, it involves two rights: the right of Allah, which is waived by repentance; and the right of the husband. The original principle is to seek his absolution (حلّ التحلّل), but the harms of seeking absolution from the husband are greater, such as the spread of indecency, the collapse of families, and it may even lead to murder. Therefore, concealment (ستر) is preferred, along with supplication for the husband and sincere repentance. It is hoped that Allah will satisfy the husband on the Day of Judgment.

The original principle is to seek absolution from the wronged party unless a greater harm is feared, as in cases of adultery, backbiting, and slander. In such instances, sincere repentance, concealment, and supplication for the wronged party suffice.