I've never heard of Schleyer, but here is a discussion of the subject that I posted on another website just yesterday. Although it concerns Yugoslavia, the legal issues are pretty much the same.
"Reprisal Execution of Hostages
A 100 : 1 ratio was imposed in Serbia by GFM Keitel after the Communists began killed German soldiers and attacked small German convoys and work parties. GFM List (OB Südost) then issued an order on 4 Oct 41 directing that all males suspected of supporting Partisan activity be seized, put in a Sammellager and used as hostages for subsequent execution. This was the policy used during the remainder of the war in Yugoslavia, although the ratio applied varied from place-to-place and from time-to-time from a low of 25 : 1 to a high of 100 : 1.
Surprise: this was completely legal under the 1908 Hague Rules of Land Warfare, the American Handbook on the Rules of Land Warfare (article 358) that was in force up to the second half of 1944, and British Army rules and regulations covering the same subject. The law then in force clearly stated that “the inhabitants of an occupied territory must not indulge in any illegal acts against the occupation power.” These same rules and handbooks also permitted an occupying army to: (1) seize hostages and place them in a railway car attached to the front of a train as a precaution against the enemy’s use of mines on the tracks; (2) summarily execute “armed bandits” (as opposed to armed irregular combatants); (3) confiscate and destroy enemy property; and, (4) seize civilians and hold then indefinitely in concentration camps.
In the final months of the war and for a brief few weeks right after the war, the Russians shot German hostages in Berlin in a ratio of 50 : 1, the French executed German hostages in a ratio of 25 : 1, and the Americans used a ratio of 5: 1, burned down several German villages in reprisal for acts against the U.S. occupation authorities and executed captured German prisoners in small batches, such as in the village of Zell near Ingolstadt in late April 1945.
In the NDH area (Croatia and BiH), the taking and execution of hostages was almost always carried out by the Pavelić authorities and by the German-Croatian Police organization that was under SS-Gruppenführer und Generalmajor der Waffen-SS Konstantin Kammerhofer. The postwar Yugoslav literature is so packed full with examples of German hostage executions and related atrocities that I won't cite any here. Everyone taking part in this discussion reads Serbo-Croat, Croatian, Serbian or whatever, so you can go to a library and read the books.
But if you really want to learn about World War II atrocities in the Balkans, then you need to take a few months and read your way through the 39 rolls of microfilm covering the United States of American v. Wilhelm List et al (Case VII), also (informally) known as the “Hostage Case” and the “Southeast Case”. The trial began on 15 July 1947 and continued to April 1948. It is available as National Archives Microfilm Publication M-893. The trial transcript appears in the first 12 rolls with the English language version of the Prosecution Document Books in rolls 21 to 24 and the Defense Exhibits in rolls 35-39. So you would need 21 rolls at a minimum. The trial covers German and Partisan atrocities in great, great detail and thereby provides both sides of the picture. To simply read the postwar accounts cranked out by Belgrade is to only read about one side of the issue because, quite naturally, they say nothing about Partisan atrocities. Some of the court exhibits and depositions reveal atrocities of such barbarity that you will think you are reading about the Thirty Year War (1618 – 1648) instead of World War II (vaginally impaling and the quartering of young Volksdeutsche girls for starters). Hundreds of photographs were also submitted as part of proceedings, but to see these you will have to go to Washington because they were not microfilmed.
After you read the trial transcript and exhibits, I guarantee you will come away with a greatly altered opinion of the atrocities committed in wartime Yugoslavia and who was responsible for the great majority of them."
The legal interpretation really hinges on the spin applied by the victor of the conflict. In this case, the Allies chose to ignore the rules of generally accepted international law and judge the vanquished (the Germans) as they saw fit. It is all part of the hypocrisy that we witness nearly every day of our lives.