I'll bet it was a raccoon, based on the cages being open. I'd bet a raccoon started the problem and maybe got chased off by something else. I know it sounds silly, but we used to have a tomcat that would chase off young raccoons. The cats might have just seen an opportunity and swooped in after the fact.
As for the cats, I wouldn't worry about the other folks comments too much. Unless somebody has lived near a colony of feral cats, I'm not sure they could understand how quickly a colony can grow and how much damage they can do. The Declaration of Independence asserts that the "pursuit of Happiness" is my unalienable right. Our chickens bring my family and myself happiness, cleaning up their carcasses and explaining the death of a pet to my children does not. I do the best I can to make sure that family/pets/livestock are as safe as they can be without needless killing, but if push comes to shove, I won't lose a wink of sleep over dispatching a potential threat.