Einsatzgruppen
After a while, Himmler realized that the Einsatzgruppens methods were not the best way to kill people as quickly and cheaply as possible.p. 197  He also found that many Einsatzgruppen had trouble killing so many people.  Some had physical and mental health problems, and others became alcoholics.pp. 52, 124, 168  Himmler made a final decision after watching a mass execution in August 1941.  He decided that shooting Jews was too stressful for his men.pp. 547–548