Why was Hitler allowed to kill Jews?

Answer 1

Most Germans were not informed about what was going on. They were told that the Jews were being "resettled in Eastern Europe" and that they would "lead a better life there". However, all sorts of rumours ciriculated, some of them fairly accurate. To put it simply, most Germans weren't 'ok with it'.

Answer 2

In terms of domestic German politics, by the time he began killing Jews, he had complete dictatorial power as the Führer. In terms of international opinion, while most contemporaneous world leaders would not choose to commit genocide against their Jewish subjects/citizens, most were still relatively Anti-Semitic in their outlook. They saw no reason to help the Jewish people and the other victims of Nazi genocide.

This attitude of indifference to genocide still prevails in much of the world, including the reaction to the Rwandan Genocide and the Genocide in Darfur.