This is just my opinion.
I grew up at about the same time as rap music was growing up. When I was very young, say mid to late 80's (very early 90's), I wouldn't have associated the word "thug" with blacks at all. I would have associated it with things that were popular at the time - the mafia. Goodfellas was released, Scarface and the 2nd and 3rd Godfather movies were out or coming out. If you said "thug" back then, then mobsters pretty much came to mind, at least in my neck of the woods.
Rap music reflected this facination with the mob, and starting in the early 90's rap artists were calling themselves "thugs" with regularity. They even used the term in the titles of their songs - "Thug Life" was an album from a group started by Tupac, he also had a song called "Thugz 4 Life" (might be off on the spelling there), Trick Daddy had a song called "I'm a Thug" which was a huge hit, getting tons of radio play. These are just ones I remember. Tons of examples exist.
So the word "thug", by much of the generation speaking out now (media included) is associated with people of color, particularly rappers from the inner cities. Bearing this in mind, I do believe many are using it in a racist way - but I don't believe it's a substitute for the n-word. I think it's just another term people are using because they think it's hurtful and they want to attack the target of their anger and cause pain. "Thug" is a way they can do that in public forums (like the media) and not get the backlash of harsh racial epithets.
The word "thug" is definately not a n-word substitute for lots of us, though. I work on and off with a guy who served time in a Max Security Prison - Florence. He was deep in the Mexican Mafia and was one of the biggest drug runners in Albuquerque before he got sent here. He's half Mexican and half Italian, and guess what he's got tattoed across his forearm? The word "THUG". I was chatting with him a few hours ago about this very topic, and he told me "Everyone's a thug in prison. Mexican's use it, Italians, Whites, Blacks - everyone."
So although some may be using it in a racist way, in their own minds - please don't assume that everyone thinks the same. Many of us out here look at the word for what it is: noun - a violent person, especially a criminal.
None of which Richard Sherman is, as far as I know. The word thug shouldn't be used in it's slang form or it's literal form to describe him.