Hardcore negative utilitarianism?

In The Open Society and its Enemies (1945), Karl Popper argued that the principle "maximize pleasure" should be replaced by "minimize pain". He thought "it is not only impossible but very dangerous to attempt to maximize the pleasure or the happiness of the people, since such an attempt must lead to totalitarianism."[67] [...]

The actual term negative utilitarianism was introduced by R.N.Smart as the title to his 1958 reply to Popper[69] in which he argued that the principle would entail seeking the quickest and least painful method of killing the entirety of humanity.

Suppose that a ruler controls a weapon capable of instantly and painlessly destroying the human race. Now it is empirically certain that there would be some suffering before all those alive on any proposed destruction day were to die in the natural course of events. Consequently the use of the weapon is bound to diminish suffering, and would be the ruler's duty on NU grounds.[70]

(Pretty cute wind-up on Smart's part; grab Popper's argument that to avoid totalitarianism we should minimize pain, not maximize happiness, then turn it around on Popper by counterarguing that his argument obliges the obliteration of humanity whenever feasible!)