Lieber argued that three functional factors distinguished a soldier, who was entitled to all the protections of the laws of war, from a guerrilla, who was not: the presence of uniforms, an organized command structure and the capacity to manage prisoners of war. Such factors trumped the more traditional consideration of a formal commission from one of the belligerent parties. - NY Times 24 Apr, 2013
Art. 52. No belligerent has the right to declare that he will treat every captured man in arms of a levy ' en masse ' as a brigand or bandit.
If, however, the people of a country, or any portion of the same, already occupied by an army, rise against it, they are violators of the laws of war, and are not entitled to their protection.
Art. 60. It is against the usage of modern war to resolve, in hatred and revenge, to give no quarter. No body of troops has the right to declare that it will not give, and therefore will not expect, quarter; but a commander is permitted to direct his troops to give no quarter, in great straits, when his own salvation makes it impossible to cumber himself with prisoners.
Art. 61. Troops that give no quarter have no right to kill enemies already disabled on the ground, or prisoners captured by other troops.
Art. 62. All troops of the enemy known or discovered to give no quarter in general, or to any portion of the army, receive none.
Art. 63. Troops who fight in the uniform of their enemies, without any plain, striking, and uniform mark of distinction of their own, can expect no quarter.
Art. 81. Partisans are soldiers armed and wearing the uniform of their army, but belonging to a corps which acts detached from the main body for the purpose of making in roads into the territory occupied by the enemy. If captured, they are entitled to all the privileges of the prisoner of war.
Art. 82. Men, or squads of men, who commit hostilities, whether by fighting, or inroads for destruction or plunder, or by raids of any kind, without commission, without being part and portion of the organized hostile army, and without sharing continuously in the war, but who do so with intermitting returns to their homes and avocations, or with the occasional assumption of the semblance of peaceful pursuits, divesting themselves of the character or appearance of soldiers -- such men, or squads of men, are not public enemies, and, therefore, if captured, are not entitled to the privileges of prisoners of war, but shall be treated summarily as highway robbers or pirates.
Art. 85. War-rebels are persons within an occupied territory who rise in arms against the occupying or conquering army, or against the authorities established by the same. If captured, they may suffer death, whether they rise singly, in small or large bands, and whether called upon to do so by their own, but expelled, government or not. They are not prisoners of war; nor are they if discovered and secured before their conspiracy has matured to an actual rising or armed violence.
Art. 112. If the bearer of a flag of truce offer himself during an engagement, he can be admitted as a very rare exception only. It is no breach of good faith to retain such flag of truce, if admitted during the engagement. Firing is not required to cease on the appearance of a flag of truce in battle.
It would seem from the above that the summary execution of civilians firing upon Bavarian troops without being properly commissioned and uniformed as partisans, would have been allowable under the Lieber Code.