If we are eating an animal that is live (alive), that means we are killing it - wholeheartedly, intentionally, purposely. This means that we will have to pay for our karma. The animal being killed most likely is not wholeheartedly dying of its own choice, therefore we have created negative karma and will have to repay it in some way, shape or form as yet to be seen.
However, it is different if the animal is already dead by another person's intention and hand. Then, when the food gets to you, it is no longer a live animal, but rather a leftover carcass. This is a different issue.
This applies not only to Buddhists, but to all life forms - human, animal, birds, insects...Everyone values their life, EVERYONE and no one wants to be forcibly deprived of life. Therefore, when someone does deprive us of the chance to live, we will be vengeful and seek retribution. That is the karma that we must repay. Nothing in life is free.