What is the ruling regarding someone who treacherously killed my father while he was on his way to Fajr prayer, knowing that he was a mujahid, dutiful to his parents, charitable, and observant of prayer?

Taking a life which Allah has forbidden, except by right, is one of the gravest evils. The fundamental principle is that a Muslim is innocent until proven guilty by clear evidence, not merely by accusations. If the accusation of murder is proven against a person, then the execution of retribution (Qisas) is not for individuals to carry out, but rather it is the prerogative of the ruler or whoever acts on his behalf. If the killer is known, his case should be brought before the ruler to be punished. It is obligatory to be keen on safeguarding the lives of Muslims and preserving order.