my first thought on this is that farmers have a far greater aquaintance and tolerance of death than most of us. They see it a lot, and dispense it frequently. I used to despatch deformed piglets to the hereafter by swinging them against a block wall - it wasn't an everyday thing, but, from time to time it had to be done, so it was done without really thinking about it. Sows that were past their best and wouldn't repay the cost of sending them to the abattoir were sent on their way with a four pound hammer blow between the eyes or a bell pistol before being winched on to a pick-up owned by the local hunt. As for vermin - well I confess that killing rats afforded me a degree of satisfaction, partly because the rats were a threat to my health, but also because they were, in a way, competition for the resources and space of the farm.
Which brings us to what one might call the narcissistic side of killing animals. Farmers, or, at least, most of them, bear a kind of myth around with them - the myth that they are persecuted by history. It's founded on their isolation and powerlessness, and the contrast between that and their unique status as 'owners' of a huge proportion of the planet. They don't do irony, so this schism between two different orders propels them in to a kind of hysterical desire to maintain heirarchies. And there is no better way to maintain a heirarchy than to shoot something.
Despite Cubist's protestations rooks do get killed at this time of year because they can be killed - there's less vegetation on trees and on the ground, and they're easier to shoot. It's a kind of seasonal celebration. Rooks are big birds that give every sign of not giving a ****. That is their mistake. And mistakes have to be paid for.