I'm a big Philly fan, that's how I grew up. Probably my favorite Philly fan story was when I went to see them play the O's at Camden. The Phils were good that year and the section I sat was predominantly Philly fans. Two dudes next to me were O's fans and the O's scored an early run and they stood up to cheer. Huge mistake. Those guys didn't make a sound the rest of the game. There was a brawl a few innings later as several other O's fans had the audacity to continue to cheer for their team in their home ballpark.
Second favorite was when I was in Philly and the Flyers were in the playoffs. Went to a local bar before the game started and asked the bartender if they were going to show the game. His response was "Dude, your in ****ing Philadelphia". He then loudly told the rest of the bar the question I asked. So, they will eat their own too.
When you grow up like around that as I did, you just expect that is how every fan base cheers. Now, when I go back up their for games, I kind of roll my eyes a bit and if I am walking in or sitting by fans with other teams gear on, I try to give them a warning, that it isn't going to fair well for them if they want to really represent.
I would never go to a Philly game and root for the opposing team. I get why everyone hates it and as a diehard Philly fan, you wear that as a badge of honor, but yes their is a segment of our fanbase that takes the drunken degenerate badge way too far. .