Farmers kill moles because if you cut silage in a field with mole hills in it it causes listeriosis in the livestock that eat it , which can kill them or make them abort .
They kill rabbits because they eat grass that is meant for the sheep and cattle
Pigeons destroy arable crops when they come through the ground .
If you have a big population of foxes at lambing time you will loose lambs to them.
Can’t think of a reason to kill a spider other than to make it rain .
There is reasoning in what you perceive as the madness , to produce food !