Because enough other beings wanted it dead, what began to answer calls on it after that point would be something that would refuse to take the old entity’s route into parasitic anti-human activity.
Investigate Amut/Ammit, the great devourer of Egyptian afterlife, which destroys the personality to the extent no form of it remains.
Also, no-one would suggest killing a human just means they will be back in their old form in 25+ years in a new body…
With respect (I mean, not to demean your question) the weary assertion heard from some quarters that one cannot kill a spirit because belief just respawns it has more to do with a crossover from gaming and the idea all spirits can only exist when believed into being, than with the accounts of both ancient civilsations and of modern-day shamanic healers who meet with spirits unknown to man, and who don’t seek any kind of worship or attention beyind performing their task. 
And people who believe that way tend to dodge these questions: “Are Demons Real?” ~ 4 Questions To Dissect.