First to clear up a term: a concentration camp is not the same thing as a death camp

• Land of the Concentration/Death Camps - /u/estherke defines terms

• How did it become common knowledge that nazi germany was killing it's jewish population? - /u/estherke provides more details

• Where did Americans think all the Jewish people were before we found the concentration camps at the end of WW2? - /u/OnkelEmil gives an overview

As for why detainees weren't killed immediately...

• Why did the nazis spend time and resources transporting Jews to concentration camps by train to gas them rather than shooting them on the spot? -- includes links to several more

• Why Were Some Jews Kept Alive in Concentration Camps During the Holocaust?

• Was the full horror of the Holocaust conceived from the beginning? When the Nazis began sending Jews and others to ghettos, was the goal already mass extermination? Were concentration camps always intended to be death camps, or was there some other ostensible purpose at first?

• I am German and I have some questions about concentration camps and the Holocaust.