Really it's the same thing that started Emily's path of revenge. Her father asked her to forgive and she couldn't. Margaux is now to Emily what Emily was to Victoria.
But now that Malcom has been killed, why couldn’t everyone clear Daniel’s name? They could spin it so that Kate went after Emily because she’s David’s “grandson’s” godmother and she has lots of money, or something. I am sure they could have cleared Daniel’s name and protected Emily’s identity if they tried. Margaux’s “revenge” is unwarranted and does not honor Daniel. Daniel died to protect her. He chose her over his mother. He wouldn’t want Margaux to hurt Emily.