How big of an influence towards not eating cheap chicken anymore should the overuse of antibiotics be in your ethical framework? Relating to "buying fair-trade coffee for the plantation worker's sake", "not driving (combustion) cars because of environmental impact", etc. I find it hard to estimate.
For me, the antibiotics situation had only sub-conscious influence; the concrete, formulated reasoning was more like this: Before becoming completely vegetarian, I started preferring big animals like beef and pork over chicken because their amount of meat per animal killed is higher. Then I stopped eating beef because of its climate implications. Finally someone told me pigs are actually quite intelligent so even though it's just few animals per kg of meat, their ethical weight weighed more heavy again. Luckily at this point it was not a big step anymore and pretty easy to stop eating meat altogether.
By completely vegetarian do you mean vegan? Because if you've come this far but still consume dairy or eggs, you might consider how the milked cow or egg laying chicken still lives a horrible life and eventually is slaughtered, too.
Edit: I'm vegan, for some of the above reasons, but also for other reasons.
For me, the antibiotics situation had only sub-conscious influence; the concrete, formulated reasoning was more like this: Before becoming completely vegetarian, I started preferring big animals like beef and pork over chicken because their amount of meat per animal killed is higher. Then I stopped eating beef because of its climate implications. Finally someone told me pigs are actually quite intelligent so even though it's just few animals per kg of meat, their ethical weight weighed more heavy again. Luckily at this point it was not a big step anymore and pretty easy to stop eating meat altogether.