My favorite "come back" and the one that works the best is....
"Well, that chicken you ate in your sandwich last night was thrown up on a metal conveyor belt upside down and then drug across a blade. If it was lucky the blade made good contact, if it wasn't so lucky it just got cut a bit or not at all. Now, the conveyor belt keeps going and allows enough time for the dead ones to drain their blood, but the living ones get to hang there and watch this while they all get closer to the large vat of boiling water that they all get dunked in before getting plucked by a giant machine that bounces them around ripping out there feathers until they are bare. Now, you see exactly why it would have been better to get your throat slit from the beginning, because some of these chickens, possibly the one that made your sandwich, was alive through most or all of that. They are called "redskins" by the industry and it happens all the time, so chances are you have eatten a "redskin" lately. Now, if you think that me caring and respecting for a bird and giving a quick, clean end to it is cruel, you just go right ahead; because personally, I think you are the barbarian for supporting a industry who does not respect the animals or you for that matter."
I also like to remind them and switch out "chicken sandwich" with "those chicken nuggets you ate took way more than one chicken and they were...."
Usually I don't get all the way through... they stop you from all the details. Point is... people like to pretend their food didn't die. I just like to remind them that not only did they die, but they never got to truely live either.