I'm not Net-thuggin' here, but a Celtic supporter threatened to stab me once and it was for no damn reason whatsoever. In his mind, it was, though. It's because I said something along the lines of "in Scotland, I like Rangers the most." Probably wasn't a wise thing to say, but we were just standing outside of a pub having a cigarette, me and my friends, and a bunch of other supporters. He overheard it and started threatening.
One shouldn't generalize, and I know there are scumbags connected to absolutely every club in football, but ... I dunno, I've just never had any positive experiences with Celtic fans. And their behavior after Celtic-Juve just pretty much confirmed, to me, that a vast majority of them are moronic, miserable cunts.