Lange was responsible for numerous crimes against humanity, including the murder of mentally disabled patients in Poland and in Germany during the Aktion T4 "euthanasia" programme, and became one of the key originators of the Holocaust.

[1] In the beginning of 1940 he assumed command of an SS-Sonderkommando Lange, named after him and tasked with the murder of mentally ill in Wartheland area (Wielkopolska) under the direction of SS-Standartenführer Ernst Damzog and SS-Obergruppenführer Wilhelm Koppe.

[6] By December 1941 Lange was a SS-Hauptsturmführer (captain) and was appointed commander of the Chełmno death camp by then SS-Standartenführer Ernst Damzog, chief of the Sicherheitspolizei (SiPo) and Sicherheitsdienst (SD) in Posen (Poznań).

At a very minimum 152,000 people (Bohn) were killed at the camp,[10] though the West German prosecution, citing Nazi figures during the Chełmno trials of 1962–65, laid charges for at least 180,000 victims.

[11] Upon the completion of his task in 1942 Lange was transferred to the Reichssicherheitshauptamt (Reich Main Security Office) and served under Arthur Nebe as a Kriminalrat (Criminal Investigator).

[12] In March 1944 Lange returned to the already inactive death camp at Chełmno, and resumed the gassing operations on the request of Arthur Greiser, for the final ten transports of ghettoised Jews.