The worst places aren't the ones you've heard of. Once a neighborhood makes enough news, people begin to care a little, and the police make at least a token visit. The worst ones are those isolated places in the city that most people don't know exist, and where no sane, law-abiding person would ever intentionally visit. That's usually how it seems to work, at least in the cites I've lived and visited.
Pittsburgh, on a whole, is a fairly safe city. There are several "bad" neighborhoods, where you lock the car doors while driving through, and where it's not uncommon to witness a violent crime in progress. Don't get out of the car after dark, don't stop if someone tries to flag you down, hit the gas if someone walks up to your car. Use a little common sense, and you'll be fine.
Then there are the Hell Holes. I was making the mile and a half drive home from a friend's house one night, and wasn't paying attention. Maybe two blocks or so from his driveway, I made a bonehead wrong turn but didn't realize it for several blocks. The road took me into an unfamiliar area; and since I wasn't sure where I made the wrong turn, I didn't know where I needed to go. After twenty minutes of random driving, I found myself in a remote part of the Lincoln-Larimer neighborhoods.
Oops.
I've never seen a neighborhood like the one I drove through, and I hope I never do again. The roads were so beat up (destroyed is probably a better word) that I couldn't drive over 10 mph without rattling my teeth out. At least a third of the houses had no doors or windows, but most appeared to be occupied. There were plenty of streetlights, but not a single one was on. Burned out cars, obvious crack-houses (and not just one or two), gang graffiti covering most walls, bullet holes in windows, walls, cars, etc...this neighborhood had it all. I saw a couple of guys openly carrying, and while I only spent maybe fifteen minutes trying to find my way out, I counted six seperate gunshots (not six shots, but six different incidences of gunfire). All in all, not a fun place to spend your Friday evening