The root of actual evil lies in Beriyah-Yetzirah-Asiyah.
The root of actual evil is in BYA, not Atzilut. Atzilut is related to evil only because BYA are its garments.
The rule of good-and-evil from Op. 30 raises a structural question: where is evil rooted, structurally? If evil were rooted in Atzilut itself, the rule of unity would never be possible (Atzilut is intrinsic to Eyn Sof's revealed Will). Klach's answer is precise: evil is rooted in BYA — Beriyah, Yetzirah, Asiyah — the lower three of the four worlds. Atzilut is related to evil only because BYA are its garments; the root of evil is not in Atzilut.
The biblical anchor is Psalms 5:5: evil will not dwell with You. Klach reads this as a structural fact, not a poetic flourish. Evil will not dwell with You — meaning that in Atzilut, where Eyn Sof's Will is most directly present, evil has no foothold. Evil dwells in BYA, where the cosmic government has descended into the operational levels suited for the cycle's middle phase.
Why BYA and not Atzilut? Because BYA are the levels at which the Vessel has assumed enough operational specificity to admit the conditions for evil — the Other Side's parallel structure. Atzilut is too close to Eyn Sof; the contrast with the encompassing limitlessness keeps evil from gaining a structural footing there. BYA, having descended further, has the operational space in which the parallel structure can operate.
The consequence is doctrinally significant. Atzilut is related to evil — in the sense that BYA are Atzilut's garments and so what happens in BYA touches Atzilut at one remove — but Atzilut itself remains free of evil's root. The cycle's repair therefore concerns the purification of BYA primarily, with the consequent restoration of the Atzilut–BYA relationship.