For as far as I have heard, traditionally (in the Vinaya and commentaries), five conditions are needed to break the precept/rule of not killing:
- A living being
- An awareness that it is a living being
- An intention to kill
- Making an effort to kill
- Death of the creature through that effort
If any of these are missing there is no breaking of the rule. I feel that when we buy or eat meat we do not have, for example, the actual intention to kill the animal. We want to eat (to nourish our bodies, or for enjoyment etc.).
I agree that this view seems rather legalistic and does not as much keep the spirit of harmlessness in mind. But my point is that the intention in what one does is really important. So if you feels that when you eat meat, you are fully responsible for the killing of that animal, then not wanting to contribute, you can choose to be vegetarian. If you feel that killing (by a butcher) is one act, and eating is another, you can make your choices based on that. The actual act of killing (stealing, sexual misconduct, lying) itself is universally unwholesome but I think the interpretation comes in to determine where one action ends and another starts. And I do feel that that can be based (at least partially) on personal view (through ones intention).
This, like I said before, is not to say that a vegetarian lifestyle is not praiseworthy if done in order to prevent harm to living beings. I just think it is a further training one can take on, not one necessarily contained within the first precept. Even though the Buddha had the direct opportunity to lay down the rule (when challenged by Devadatta), he did explicitly not do so. To me that seems to show a difference between killing a living being and the eating of meat.
Dry up what pertains to the past,
do not take up anything to come later.
If you will not grasp in the middle,
you will live at peace.—Snp.5.11,v.1099 (tr. Ven. Bhikkhu Bodhi)
Whatever is will be was. —Ven. Ñānamoli, A Thinkers Notebook, §221