Reply To: Praying About Past Events and Parshas Sh'mos.

your question has nothing to with Hashem changing the past or the future but rather it has to do with the machlokes rishonim if a person can kill another person that is not predestined to die or since he killed him that was predestined the ohr hachaim in vayeishev says that people can kill someone who is not predestined to die but animals cannot therefore reuvain wanted to save yosef from the brothers but did not care about the snakes and scorpions in the pit here as well moshe was killing a mitzri that was not predestined to die so if he would had future descendants convert he would have let him live since people have bechira they can change someone elses destiny but without this it would be predestined so therefore if Hashem doesnt have to he keeps it predestined unless He decides otherwise