I don't think Roy Williams ever considered himself dirty, but....
When you injure a couple of players, get a rule named after you, and continue to make the same type of tackle despite penalties, I can see why other players would think that.
Also, I'm sure WR's think Roy is dirty because when they are helpless, they are the only players they've seen him hit. When it comes to RB's, Roy is a notorious ankle/knee diver...for a big hitter, he has never really wanted to take on RB's who hit back.
I still remember the Thanksgiving game against Miami with Ricky Williams...saw Roy Williams came up hard in run support again and again, but Ricky Williams never once did anything but head straight for him...and every single time Roy went to the turf with a flying ankle tackle to take him down...not once did he ever throw a shoulder into him.
I can see why RB's would start thinking he's dirty too....you see, sadly, despite all his notoriety as a big hitter, Roy doesn't really seem to like contact in the NFL....think back to most of the guys he blew up early in his career...it was always a WR trying to catch the football or a RB someone else had already stopped...then Roy would fly in and cause a fumble. He was aggressive and threw his body around when he was younger, but he was always diving into their legs.
Roy was still an above average SS, but I think his reputation for being a huge hitter was overblown.