Yet even so, out of Arich emerges Zeir Anpin… This is clearly what happens in the chain of development, as explained in the Idra Zuta (292a): “Zeir Anpin depends on and is unified with Atika Kadisha (=Arich)”.
…which is rooted in the mystery of Justice… Justice is the mystery of the as yet unpurified Residue, which functions and rules in the mode of Justice until the Line gains sway over it, after which Zeir Anpin will turn back and bestow only good like Arich Anpin. Thus the Attribute of Judgment turns back into the Attribute of Mercy.