When I learned that the Death Camps were not deathy at all, but our own post-war POW detainment facilities existed to Kill Them All...
Too energy-wasteful. How about a big barbed-wire fenced-off camp, with no food or water. They go in and never come out. No extra resources required to maintain. Basically a Death Camp, but without the beds, food, clean water, hospitals, orchestras, schools, swimming pools... just barbed-wire and no egress.
Kinda like what Eisenhower used to murder the surrendered million of the German Army.