Why militaries have their own judicial system?

In most countries, cases against members of armed forces (i.e. military) are dealt in their own court (often known as court martial). Why military personals are not dealt with normal civil/criminal courts?

This originates from that fact that many actions which are deemed right for military are considered wrong for normal people – notably killing of other people. During war, military persons are expected to kill members of enemy forces. It is part of their jobs. However, if military unlawfully kills people, then they are subject to court martial. If military issues are dealt in civilian courts, many cases will be exposed to general public and people will invariably start comparing outcomes of military cases with those of pure civilian cases. For example, a civilian murders might claim that he has been given life sentence for murdering just one person where as a military person went scot-free even after killing innocent civilians in another country. These sort of cross allegations can cause havoc with national politics and with the psychology of members of armed forces. For this reason, military court is separate from civilian courts. It is to be noted that if a military person commits crime while not serving in military, that case may be dealt in civilian court.