Law against slaughtering a pig at a suburban residence inside San Antonio limits?

So a while ago at my subdivision which is inside San Antonio city limits, some people down the street brought a live pig to their house and killed it in their front yard by hitting it on the head. They didn't know what they were doing and it took them a long time and many blows to actually kill it, and it was squealing horribly. I'm reasonably sure that it is illegal to slaughter a farm animal at a residence where you are not allowed to keep farm animals, but I was hoping that someoe could tell me were I could find the written law or ordinance.