Not that surprised to be honest. Outrage culture has been steadily leading to increased threats of violence for a while now. People have no problem seeing people they don't like as less than human and deserving of violence and harm.
Canuck fans are mad at our history and everyone is desensitized to open threats of violence that they justify in their minds. Canuck fans are upset at the state of our franchise, they like they've been wronged by Matheson, and they skip all the normal levels of being pissed off and go straight to threats of violence because that's how you show you're really really angry for realsies. People want justice, their version of justice is doing harm to those they feel harmed them, and if nobody else does it they will threaten to do it themselves.
It's too bad this is bleeding into hockey, especially for things like this rather than losing the cup final (though that still obviously isn't okay), but I think it will just get worse. This won't be the last time this type of thing happen. It's interesting how everyone can rationalize it in their minds.
"It's disgusting that Matheson injured someone, so I'm going to threaten to intentionally hurt him. I have the morale high ground here! I'm just doing justice!"
These people will not take a step back and think about what they're doing or the harm that it can lead to.
Also what kind of person threatens a dog. I can understand humans rationalizing violence to other humans, enough though again obviously it's wrong. Doggers are pure and innocent, someone who is willing to hurt an uninvolved person or creature just to cause suffering for the person they feel wronged by is pretty disgusting. Even worse than threatening violence directly, and that's saying something. These people really need to take a step back and think about if this is the kind of thing a good person would be doing, but they're so sure they're on the right side they wont even question themselves.
Dangerous stuff and it won't stop. I wish there was a way to make people stop and really consider if it's really a good idea to threaten people because they know they're right, as if some of the worst things people do, or have done throughout history, aren't completely justified in their minds. Luckily I don't think anyone will actually follow through, it's much easier to talk a big game on the internet but everyone expects someone else to do the dirty work for them. Wouldn't want to miss the next episode of sportsball or the finale of Game of Thrones.