IMO this is a systemic problem in the NHL resulting from the "let them play" mentality. The NHL and the referees have chosen to intentionally allow minor transgressions to go unpunished, which results in the players pushing the envelope a little further to see where exactly the line is, how much they're really allowed to get away with.
The thing about that though is, it isn't the player's fault. Imagine being in their shoes, you're paid millions of dollars and more than that you care passionately about your teammates and the fans around you. You're looking for any little edge to get ahead in the game, to help your team win. So you hook a guy when you know it won't be called, that player gets more pissed off and slashes you next period, so you 'finish your check' and hit 'em late. We saw an example of this exact thing with the biggest suspension of this years playoffs so far, the Maple Leafs Kadri. He saw a Bruins player elbow Marner in the head, which pissed him off, so he delivered an illegal hit to that player as payback (and admitted to this reasoning). The refs had an opportunity to blow the play dead and call the elbow but they let them play instead, and things escalated.
This begins and ends with the league choosing not to call penalties. If they want it so you're allowed to hook and slash in the playoffs, fine, write it in to the rules and have everyone on the same page. But if the NHL chooses to continue allowing all of these 'unwritten rules' determine how calls are made and intentionally refereeing the league differently game by game, we'll continue to deal with this for a very long time.
Just my 2 cents.
