Brideshead Revisited was completed in June 1944, one year before the end of the Second World War in Europe (Victory in Europe Day was 8 May 1945).
Hooper's remark implies an awareness of the atrocities (including killing in specially constructed gas chambers) which took place within Nazi concentration camps, although the full extent of these atrocities did not become known to the British people until the Allied and Russian forces began to liberate the camps from mid-1944 onwards.
Concentration camps existed in Germany as early as 1933, and were initially used to house political prisoners and opponents of the Nazi regime. Eventually, they would be used mainly for the incarceration and extermination of Jews, but other groups, including Soviet prisoners of war, Roma (Gypsies), Poles, Jehovah's Witnesses, Catholic clergy, homosexuals and people with physical and mental disabilities, were also sent to the camps.
From 1933 onwards, Germany operated policies of 'racial hygiene' based on the ideas of the Eugenics movement which sought to suppress certain characteristics in the population through the forced sterilisation of those carrying 'undesirable' genes. Between 1939 and 1941, in a program known as 'Action T4', Hitler authorised the killing of over 70,000 individuals considered 'incurable', including many patients in mental institutions.