This spread mechanic is really great QOL for Damned and Poison builds while it is nothing for single target, I feel it is almost mandatory for clearing.
But it is a global ability to spread damned/ poison when you inflict an enemy with Mark for Death. Its not specific to the mark for death on Profane Veil’s Grave circle node. It can be triggered by Mark for death on many other sources (such as Bone curse, Volatile Zombies, the minion idols, Zerrick’s set, Mortality’s grasp, etc.).
So why is this global buff(which does not rely on using Profane veil in any way) kept within Profane Veil’s Skill tree? This node would make much more sense as a Warlock Passive IMO.