The Nazis did not "have to" circumvent German human rights laws by taking people "abroad".
There were many concentration camps in Germany where Jewish people were killed.
The main extermination camps were in "Poland", but, Germany annexed much of this territory making it part of Germany. So, for example Auschwitz and Chełmno are now in Poland, but were in Germany then.
Also aspects of The Holocaust developed out of Aktion T4, murder via involuntary euthanasia of those deemed 'genetically deficient'. This occurred in Germany too.