Oofta
Legend
Wat.
You seem to be inferring an awful lot that the rest of us aren't getting out of this, jgsguden.
He's reporting on a bad play experience. This is useful fodder for discussion. Given his comments about how it derailed the last hour of the session and one of the other players was exasperated enough to walk out of the room, it does sound like Zardnaar could have found a better way to handle the situation, but I don't see any bragging or abuse here, and your comments about cruelty and rudeness seem completely unfounded.
I'm more sympathetic to the folks suggesting parting ways with the player. While we don't have full context to really judge how stupid or bad their play was, and how well Zardnaar signposted the danger, details like the player telling Zardnaar he wasn't going to take Con saves for his Concentration spells unless Zardnaar prompted him do sound like it's a bad/"abusive" player.
It was probably unintentional, but the tone of the posts conveys a message. The OP didn't like the PC class, the player was being annoying and obstinate while challenging the DM's authority so the DM killed the PC after creating an ad-hoc challenge that was practically guaranteed to kill the PC. It was okay because the player knew the name from the MM so he could have known* what they were up against. It's the player's fault for being so uppity and annoying.
On the other hand, I was not there. If I have a problem with a player (and there are some real red flags that have shown up after the OP) I'll talk to the player, not "teach the player a lesson" by killing the PC. If someone refuses to follow the rules of the game they can find a different table, but that's a discussion that happens outside of game time. If the player challenges a foe that will kill them to a fight to the death I'll do a quick sidebar and timeout to clarify with the player what is going to happen.
The result may have been similar at my table, the path there would have been different.
*even though the player is new and may have never seen a MM