I am currently the proud owner of a pit bull. Yes, I say proud, because my dog is the nicest, sweetest, most lovable dog I've ever owned. He gets the shit kicked out of him by my other dog...and that dog is half chihuahua, half pomeranian. Yes, he plays, rough, but the only time I've heard of him being aggresive is when a group of dogs at the park surrounded him. and no one was hurt, no dog was bit, he just growled enough to get them away. And you know what? I was also smart enough to not bring him back there...why? because I don't want to risk it.
The pit breeds were all bred as hunters and fighters. They are naturally aggressive. But they are not naturally killers. When treated poorly, their aggressive tendencies get more extreme. When not trained properly, they are slightly, slightly, more wild than the average dog. But they are NOT evil nor are they murderous, with only killing on their mind. Its unfortunate, but some scum sucking pieces of shit give these dogs bad reps. No dog should be a fighter. Thats horrible.
Also, the guy with the pit pulling cinder blocks: there are contests for pit bulls called weight pulling contests. Sounds simple enough right? Whatever dog is the strongest, pulls the most weight, and wins. No fighting, no harm done (the weights are supposed to be on wheels). So he may have been training for that, though he should have had a harness and like i said, wheels for the weights.