At What Point Between Human and Zombie Does it Become Ok to Shoot?

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In The Walking Dead, nobody hesitates to blow a zombie's brains out. There's no debate about whether it's ethical, whether zombies are some new form of life that we don't have the right to exterminate blah blah. People just point, shoot, then bury or burn the remains.

Likewise for 28 days later zombies, even though they aren't corpses. These are living, breathing people who are simply violent and irrational because of a brain parasite. Yet nobody hesitates to shoot them.

Is it okay to shoot them if they are alive, but infected by a virus that makes them mindless and aggressive? Is it okay if they are undead, but still conscious and intelligent? Where is the line? What degree of "zombification" makes somebody no longer human, and thus ok to kill on sight?

What if zombies were alive and intelligent, but still under the control of something which compels them to pursue the uninfected and spread it to them?

For example, I wrote a pair of novellas in which humanity is rapidly assimilated by a sort of mechanical parasite. It's simply a device you wear on your neck.

It is marketed as a "wellness" and mood improver, like those new agey holographic or magnet bracelets that were a fad a few years back. What it actually does is to reinforce with dopamine all thoughts and actions which cause the host to seek out and place more of the device on people who don't have one on them yet, and makes them violently hostile to any effort at removing it or inhibiting its spread. (It also kills them if successfully pulled off.)

Is this type of zombie ethical to shoot? They are intelligent and for the most part retain their personalities, they are just under the control of something they don't realize is influencing them and it cannot be removed safely. It will also eventually overtake every human on Earth if not stopped.

I think any degree closer to human, and we would agree it's unambiguously wrong to shoot. Brain slugs from Futurama for example, since you can easily remove them and the person immediately returns to their former self.

But where is the line drawn, before which you can shoot, but after which you cannot?

^{Stay Cozy!}