Karma: Intention vs knowing result of an action
The difference is that it is possible to drive a car without the intention of killing the bugs you will hit, but it's not possible to kill an animal for a particular purpose without intending to kill it. You can't do something for a specific purpose without somehow forming the intention to do what you are doing.

For example if a farmer decides to kill a chicken in order to cook it and then actually kills it, then at the very least when he is preforming the act of killing he has the intention of killing. If he didn't have the intention of killing it then the physical act itself couldn't happen.