Why are pitbulls bad dogs?

Don't think there's any breed that's genuinely bad as such.

However, pit bulls are the favourite breed for people who thinks that dog fighting is a good hobby.

And if you breed dogs to fight, and then train them to fight, well, then you do get some rather nasty creatures eventually.

You can just as easily have good pits. Might take some generations of breeding and/or a lot of upbringing to curb that fighting instinct. But perfectly doable.