First of all, I have no sympathy for the Internet death threat issuers. They should be tracked down and dealt with.
Now onto Mike "The Pylon" Matheson. If he doesn't want to face anonymous online tough guys, don't go WWE in the first place. Hundreds of NHL players have bad games every week, he was lame enough to let his frustrations overcome his rational sense.
He is a professional and paid millions for his job, but his behavior was wholly unprofessional. If I, or anyone else got frustrated with a lousy cup of coffee, and in the heat of the moment punched out some Tim horton's employee and caused him to get a concussion, and if it were on camera and watched by hundreds of thousands, you'd better believe people would get angry. Does not justify the reaction by the anonymous Internet gangstas, but it does not absolve the initial crime either.