Why were Jews killed in concentration camps?

Hitler made the Jews the scapegoat for everything that was wrong in Germany and convinced enough people that they were taking up space that would be better used by his "Master Race" of Aryans; or if you really mean why it was done in the camps instead of somewhere else, that was more or less a business decision - mass executions were more efficient. It also hid the truth to some extent. People knew that their neighbors had been taken away to a "camp" but in the beginning very few people knew the true purpose of these camps.

Slaughtering Jews publicly, in the streets, in full view of the public, might have aroused sympathy. It would also have been a form of public disorder - something the Nazi leadership disliked. This happened in Bucharest, the capital of Nazi Germany's ally, Romania, the Nazis ordered a halt to public killings. After all, it made everything just a little too obvious.