Germany certainly did have conscription and a good many of the soldiers of the Wehrmacht were conscripts. Some branches and specialties had a high proportion of volunteers; Luftwaffe pilots for instance were largely volunteers (although all Germans with a pilots' license were drafted in 1939, which led to Germany's few women pilots like Beate Uhse to become Luftwaffe auxiliaries) and the SS prided itself on its volunteer status. But the Wehrmacht was not a slave army or one that destroyed free will. The German armed forces, like its other contemporaries, was a modern force that had an organized system of military justice. German military law had much of the same language and responsibilities as Anglo-American military law when it came to criminal orders. §. 47 of the Militär-Strafgesetzbuch für das Deutsche Reich stated:

If carrying out an order in the course of duty should violate a law, only the superior who gives the order is responsible. However, the subordinate who obeys it is punishable as a participant: a.) if he goes beyond the given order or b.) when he knows that the superior's order would have the aim of leading to military or other crime or violation.

This regulation dated back to the Kaiserreich and remained in force during the Third Reich. Like Anglo-American contemporaries, the onus of legal responsibility was largely on the commanding officer giving the orders, but section b. of §. 47 was actually more strict than its contemporaries in that it implied a subordinate could disobey a superior's orders if they were criminal.

German military authorities were aware of §. 47 and its implications for destroying racial enemies of the state, but either the Germans ignored these restrictions or crafted their own orders that declared them null. The most salient example of the latter was the Barbarossa Decree of 13 May 1941 in which OKW declared that there was “no compulsion to prosecute,” criminal offenses committed by Wehrmacht troops against enemy civilians unless such offenses directly hindered German military objectives. Other decrees like the Commissar Order sidestepped procedures for military criminal justice in favor of more violent and immediate measures. However, despite these various edicts, the legal mechanisms of German military justice remained intact. The historian Christopher Browning has found that of the decidedly small minority of Germans who did refuse orders to commit war crimes, none suffered any punishment or court martials. /u/commiespaceinvader has an answer of theirs that distills Browning and other historians' analysis of participation in war crimes.

The problem German war crimes presented to military justice was that they were of a different species of unlawful orders that then existing military law was ill-prepared to handle. Most legal jurisprudence on unlawful military orders were predicated on the idea that the offending officer was acting in a rogue manner and violating the laws of war. These violations could either be for one’s personal enrichment or fighting their own kind of war such as shooting prisoners or using illegal weapons like gas. This made German war crimes problematic because these orders were not counter to state policy, they were state policy. Orders to execute civilians and other genocidal acts emanated from the very upper echelons of the German state. Most of the corpus of existing military law treated such unlawful orders as the exception, but in Germany’s war, they were the rule. This was one of the reasons why postwar military laws had to be rewritten extensively to accommodate what to do when a state's leaders violate the rules of war and its soldiers carry out their orders.