I didn't read past the OP.
My opinion is simple-- you failed as a DM and any veteran players at the table failed as veteran players.
Its quite easy in the role of a DM to kill off a player. Trivial, really. You can just rule any trap or whatever to be autodeath. You can throw in an impossible to defeat NPC/monster at any time. It is absolutely trivial.
Thus-- killing off a player for not playing D&D "the right way" is nothing to be proud of or brag about.
You control the game, the story, what happens at all in reaction to what a player does. Now-- I do understand that this player missed session 0, and maybe that is on them, but in the very least you should make sure that this player's expectations for what he expected to happen in the story met what the expectations of everyone else at the table expected from the story.
Honestly-- in any given fictional story, the protagonists of those stories are often rewarded for whatever decisions they make, are just simply vindicated in their actions-- because the same person is writing the character's actions and the results of those actions. Take any random action movie, whatever it is-- James Bond, Jason Bourne, John Wick, Fast and the Furious-- and you will see protagonists taking drastic actions that results in the deaths of others and, if the plot decided they should be held to "realistic consequences", then they would probably fail and die in one of the stupidly dangerous things they did or it would be revealed they murdered someone undeserving for no justifiable reason and the justice system would hold them account for it.
That sort of action movie vibe isn't what you were going for? The player was to be punished for sticking relentlessly to their character's strict code of violence? Okay-- it was kind of your role as the experienced person in roleplaying to explain this. Play the "Yes, and..." or "Yes, but..." game with them and encourage them to do the same for the rest of the players. Have a conversation and instead of "I hate you, you do everything wrong..." try a "Everyone would enjoy things more if you..." conversation.
Now, I totally get that you were angry in that particular moment-- that you felt you were giving them too much of the time and letting them go off on too many tangents and you tried to be patient... but, thing is-- did you really properly communicate this?
If you came here expecting to be applauded for killing someone's interest in roleplaying because they were fulfilling their role of an idealistic prick all too well in a game you expected to be all grimy and underhanded and compromising... maybe you will find some of that here, and that is sad.
But most of us-- we kind of want to keep the hobby alive. And driving players away from the hobby because the screw up in their first few times playing is just not the way to do that.