why doesnt frankenstein take the blame from justine

I always thought he blamed himself for the murder. Rather than blame the monster for his downfall, Frankenstein blamed himself because he created the monster's life. Frankenstein felt as if he murdered Justine as well as William because she was executed for a crime the monster committed. Elizabeth was altered by the injustice of Justine's death, and Frankenstein felt responsible for that alteration as well. The chain of events that the monster set off with William's murder began not with the monster, but with Frankenstein's desire to create life.

Oh, I see what you mean. Victor simply didn't want to incriminate himself. Part of him wants to be free to destroy the monster and part of him really doesn't want to go to jail.