Someone could be accurately described as a parasite at any stage of their life.
A parasite to their spouse.
A parasite to society.
A parasite to the planet.
So, by your logic, we can kill anyone who becomes a parasite.
I suspect that you would not want to be instantly killed if you fell into a coma.
If I were you
And I wish that I were you
All the things I'd do
To make myself turn blue