The big difference there is, Dallas is known for getting players over the year with character issues, so it is not a big story cause it's the same old, same old with them.
Meanwhile, the Patriots have this rep for being smart when it comes to getting players with issues, and considering they gave such a massive contract extension to a guy with the background and problems Hernandez had, the criticism in that regard is more than justified. No, they couldn't possibly know that he would eventually be accused of killing someone in cold blood, but his gang background has come out now, and there is no way they didn't know about that, plus way more stuff than we the fans do. In short, they gave big money to a major risk, and it backfired.