Mario Williams was released by the Bills for reportedly being a locker room cancer. If there is one thing that John Harbaugh will not tolerate, it is the unity go the team being challenged by a singular player.
After all, his motto is 'the team, the team, the team". No one is bigger.
Now if Williams was actually a good, consistent player maybe and perhaps only maybe, would Harbaugh even entertain the idea of bringing him on. But even then, it would be safe to assume that Williams would demand elite pass-rusher money as he has in the past, signing the largest contract ever for a defensive player (at the time of the signing.)
At this point, Williams is more of a run-stopper then a pass rusher. Even the run stopping aspect of his game could be disputed as he only posted 19 tackles this season.
Williams is the type of player who if asked to set the edge, will set the edge; no more and no less. Going above and beyond is not in his vocabulary.
Then comes the whole 3-4 and 4-3 issue. While the Ravens don't run the pure 3-4 Rex Ryan does, they do deploy their outside linebackers from the two point stance quite a bit, especially on early downs.
The Ravens aren't a fit from Williams' point of view, and the latter is not a fit from the Ravens' point of view. Let's just turn around and walk away.