How does God forgive the killer of a hundred souls, yet not forgive one who has taken a right from others without returning it? And will the ruler who kills, commits فسق (moral transgression), and betrays, then repents before his death, be saved?

Allah forgives all sins except shirk (associating partners with God). This includes the killer of a hundred souls, if He wills, He may forgive him.

As for the martyr or one who died owing a debt but intended to repay it, Allah will pay it on his behalf.

Ibn Abbas issued a fatwa to the angry questioner stating that the repentance of a deliberate murderer is not accepted, in order to deter him. Otherwise, he knew that Allah forgives everything short of shirk.

Whoever wrongfully takes people's money commits a sin, and the rights remain a burden on him. His affair is with Allah: if He wills, He may pay it on his behalf and admit him to Paradise; and if He wills, He may hold him accountable. The one who comes on the Day of Judgment with good deeds, but they are taken from him to compensate those he wronged, is declared bankrupt. Then, if his good deeds are exhausted, he is cast into the Fire.