I must say I'm with Prometheus on this one. Pit bulls are the result of genetic engineering. They are a human creation, bred, as P. says, for aggression. They wouldn't exist if they hadn't been engineered into existance.
Also, when people talk about getting rid of a breed they don't mean rounding them up and killing them, they mean preventing them from breeding and therefore discontinuing the line so to speak. So no cruelty involved.
That's not to say they can't be perfectly nice dogs. However, they were bred solely as fighting dogs. It's hardwired into them.