T - it's a very mucky area of it....but it's not the GENEVA Conventions that apply, it's the Hague Conventions, specifically the HRLW, the Hague Rules on Land Warfare, or Hague IV
http://avalon.law.yale.edu/20th_century/hague04.asp
1. First of all - the laws and customs of war are VERY clear on what classes of civilians CAN legally bear arms...my empasis in places -
Article 1.
The laws, rights, and duties of war apply not only to armies, but also to militia and volunteer corps fulfilling the following conditions:
To be commanded by a person responsible for his subordinates;
To have a fixed distinctive emblem recognizable at a distance;
To carry arms openly; and
To conduct their operations in accordance with the laws and customs of war.
In countries where militia or volunteer corps constitute the army, or form part of it, they are included under the denomination "army."
In other words - under certain conditions civilians CAN take up arms
WHILE A WAR IS GOING ON... This is important to understand - because once a war is
over and a nation has been "Occupied", military action by civilians in
ANY circumstance is
NOT permitted under the Rules, in the sense that there's nothing specifically
against it -
but there's NO LEGAL PROTECTIONS for anyone fighting as a partisan!
The Occupiers ARE free to execute partisans after a minimum "court martial" to confirm they were/were not guilty of whatever action they're being accused of. Military laws apply I.E.
those of the Occupying army; partisans have NO rights to be put into the civil law system of an Occupied nation and tried for murder.
2. The execution of civilians in reprisal.
This is where things get even more confusing . Yes, it IS permitted, in that most combatant nations' military regulations, quoting the HRLW, told their officers and men it WAS ok to shoot civilians....
but NOT in "pure" reprisal; the legal way was to take x-number of "hostages" and give x-amount of time for their compatriots to hand over or identify the partisans in their midst - and if this is NOT done
THEN shoot the hostages.
There was a catch! The NUMBER of hostages you could take and shoot. There wasn't a specified limit

BUT it was quite clear that
you weren't supposed to overdo it! 
And this principle was enshrined in the outcome of the now-legendary "Hostages Trial"
http://en.wikipedia.org/wiki/Hostages_Trial...
...and it's important to note that THIS is the point in time when the customs began to change, because of the outcome of this trial and
all the legal protections afforded "francs-tireurs" were later written into the 1949 Geneva Conventions to protect partisans AFTER an Occupation was in place and the actual "land warfare" bit was
over!
THIS is the bit of that Wiki entry that's relevant to the SECOND WORLD WAR treatment of partisans...
The tribunal had to deal with two pressing questions:
were partisans "lawful belligerents" and thus entitled the status of prisoners of war?
was taking hostages and reprisals against civilians as a "defense" against guerrilla attacks lawful?
On the question of partisans, the tribunal concluded that under the then current laws of war (the Hague Convention No. IV from 1907), the partisan fighters in southeast Europe could not be considered lawful belligerents under Article 1 of said convention. On List, the tribunal stated
"We are obliged to hold that such guerrillas were francs tireurs who, upon capture, could be subjected to the death penalty. Consequently, no criminal responsibility attaches to the defendant List because of the execution of captured partisans..."
So -
as of WWII...partisans were francs-tireurs...
but ones who were OUTSIDE Hague IV Annex Article 1.
Obviously - though legal, this was unsatisfactory!

So was goin' to change bloody soon!
On the matter of killing
HOSTAGES, however, the court found AGAINST List! But what is important is
WHY...
Regarding hostage taking, the tribunal came to the conclusion that under certain circumstances, hostage taking and even reprisal killings might constitute an allowed line of action against guerilla attacks. In the tribunal's opinion, taking hostages (and killing them in retaliation for guerilla attacks) was subject to several conditions. The tribunal also remarked that both the British Manual of Military Law and the U.S. Basic Field Manual (Rules of Land Warfare) permitted the taking of reprisals against a civilian population. (The British manual didn't mention killing, the U.S. manual included killing as a possible reprisal.) Nevertheless, the tribunal still found most of the accused guilty on count 1 of the indictment because it considered the acts committed by the German troops in excess of the rules under which the tribunal considered hostage taking and reprisal killings lawful.
In other words - it WAS legal to kill them....what List and co. had done was take and kill
TOO MANY!
As you can see - that court judgement - made on the basis of the laws and customs of war
as they stood then...threw into
very sharp relief some of the contradictions in the old Hague Conventions

That is WAS legal in various circumstances to murder civilians!
And THAT is why within a very short time -
things changed.
"Well, my days of not taking you seriously are certainly coming to a middle." - Malcolm Reynolds