He believed in making a new "perfect aryan race". Jews didn't fit that description, so he decided to kill them. He killed anybody really, but Jews were his main target as he was very anti-Semitic and blamed them for basically all of the worlds problems.
The Crusaders attack the northern part of Jerusalem because it was the closest part of the destination to them, and to achieve 'surprise', it must be quick, and so they couldn't wait out a siege... hope this helps
I think the 3rd answer is correct
Possibly, though I can't really say.
