Evil was rooted at the end of the vessel, so that when the light reached there, the vessel broke.
Evil's root is at the end of the vessel — Malchut, the feet of Atzilut. When the light reached this end-place, the vessel could no longer contain it: the breaking happened.
The chapter also reveals why the breaking happened where it did: at the operational extremity of the cosmic structure.
The cosmic vessel — the Sefirot of Atzilut, considered as a coherent structure — has a bottom: Malchut, the operational feet. When the Kav's light enters from above and descends through the Sefirot in succession, it eventually reaches Malchut. At Malchut, the cosmic gradient is at its steepest: the maximum descent from the source's limitlessness to the operational specificity of the receiver. The Vessel's containment-capacity is most strained here.
Klach reads Genesis 36:31ff as the biblical anchor: the kings of Edom who reigned in the land of Edom before any king reigned in Israel. The ARI's tradition reads the kings of Edom as the Sefirot-of-Tohu — the seven lower Sefirot in their pre-repair state, who reigned (had operational existence) before any king (i.e., before integrated rule) was established. The fall of these kings is the breaking of the vessels.
The structural point is that the breaking happens at the end of the vessel — exactly where evil is rooted, exactly where the gradient is steepest, exactly where the Vessel's capacity is most strained. The location is not accidental; it is the operational form of the law of gradation Op. 44 named.