Vegetarianism for me is just a lifestyle choice that I don't impose on anybody else, least of all my pet dog. I became a vegetarian because I don't think eating animal protein is healthy. The opposite is true for my furry friend. Meat is an essential part of his diet and I think it would be cruel to allow my personal opinions effect his well being. But what if I was a strict vegan and thought the killing of animals was the worst kind of cruelty?
How do I reconcile
A) Denying an animal (which you are in control of and responsible for) it's most basic dietary requirements.
with

The systematic (though necessary?) murder of defenceless animals.
A is dependent on B
B is necessary for A
This isn't intended to be a kind of ethical paradox. I expect it's been addressed somewhere before but I haven't come across it.