Unlawful killing means taking a protected life without a valid legal right; the qualification “without right” matters. False accusation of a chaste person refers to a specified accusation for which Islamic law prescribes particular proof and legal consequences.
These examples show that grave sins include violations of other people's rights and public security, not only failings in private worship. The gravity of an act does not close the possibility of repentance, but a person's claimed repentance does not by itself erase another person's claim or automatically cancel legal obligations.