What is the ruling of Islamic law regarding someone who fails to attend Fajr prayer in the mosque due to fear of 'afarit (jinn/demons), knowing that a murder occurred near his house, and do 'afarit truly appear?

If what you mentioned is merely an imagination or a delusion, then it is not a valid excuse that permits missing Fajr prayer in congregation; rather, it is one of Satan's stratagems. Congregational prayer in the mosque is obligatory for an able-bodied man, unless there is a genuine excuse. The fear that permits absence must be real, not merely an illusion. Therefore, what you mentioned does not permit missing congregational prayer, and you must maintain your morning and evening supplications to protect yourself.