Is it permissible to harm a defendant who denies an accusation until he confesses to it, in the case that leaving the matter to his confession would lead to the loss of rights? And what are the legitimate (Shar’i) ways to deal with this situation?

If the sin relates to the right of Allah, the principle is to conceal the sinner. However, if it relates to the rights of people, such as an accusation of theft, the accuser must present evidence. If no evidence is found and the accused swears an oath, he is to be released. Some scholars are of the view that if there is strong circumstantial evidence for the accusation, it is permissible to torture the accused to make him confess to the truth, based on the action of the Prophet, peace and blessings be upon him, with the uncle of Huyayy ibn Akhtab. However, the determination and implementation of these punishments fall to the guardian of affairs (ruler) and not to individuals.