Unfortunately I wasn't able to find video, but the play stands out for me because the slash was so vicious: Bergenheim, one of the few players fast enough to keep up with Karlsson, was pestering him through the defensive zone, and forced Karlsson to lose control of the puck. Karlsson was obviously frustrated by the play, and turned around and swung his stick across Bergenheim's forearm. Bergenheim went down and looked like he was in quite a bit of pain, but he returned later in the game.
I'm not at all surprised that the fine came. It was an ugly play... I shook my head when I saw it. Karlsson had been frustrated by the aggressive Panthers' forecheck all night, and it finally boiled over with his slash on Bergenheim. He's going to have to find more constructive ways to deal with that kind of pressure, because you can be sure that opposing teams will have that particular sequence on repeat when they're preparing to go up against the Sens.