"Some may question your right to destroy ten billion people. Those who understand know that you have no right to let them live."
— Exterminatus Extremis
In Warhammer 40000 Exterminatus is the most extreme method of dealing with some form of heresy, corruption or alien infiltration. It is only used in the most extreme of circumstances, when the level of corruption a world bears is so monumental that it cannot be wiped out by any other means.
The destruction of the entire population is deemed necessary to prevent the contagion from spreading further.
Keep calm and declare EXTERMINATUS - Warhammer 40K Imperial liturgy by geektradingco