Because in the Kali-yuga they are so fallen, even they are demons, they are third-class, fourth-class demons. [laughter] [laughs] So they need not be killed. They are already killed. Their mode of life, their atmosphere, they are always dead. Just like we see so many demons. So what is there to kill them? They are already killed. They have no life. So this killing process in Kali-yuga is to kill their demonic propensities. And how to kill that demonic propensities? By spreading Hare Krsna movement.

(From Srila Prabhupada's class on Bhagavad-gita 7.2  — March 10, 1975, London)