why is Kristine willing to risk everything for Krogstad?"

simply because kristine feels that her life is unspeakably empty and needs someone to live for. in addition, krogstad and kristine were already in love, but kristine could not marry him because she had to marry a rich man so that she can afford her bedridden mother and helpless brothers comfort.when the rich man dies, she is willing to do anything for the sake of krogstad in order to save her life from the cruelty of society. we should not deny that krogstad is willing to risk his life for kristine too.