There is no, nor can there be, any *moral* justification for the estate tax, excepting an unstated "right" of the state to reach into private property and *ex*-propriate it -- as it, *take* it -- as in *steal* it. Such a tax is not based on any purported "needs of society" -- else there would be no need to wait until the victim passes -- like any user fees or consumption or income tax, it would be enabled in an ongoing fashion. No, a death tax is based not on We the People self-burdening for The Common Weal, but on a lack of respect for private property, and a greed there from, and from some unlucky lever of power being procured and pulled, to take away from someone else, "cuz they have *too*much*."
That's a few semesters of teaching Public Finance right there: when you violate the principle of private property -- or of Res Publica -- you're, eh, fucked.