How to handle a player that cannot be convinced his actions are a problem for both GM and party

First I will try to give a quick but comprehensive background information for this question.
I apologize in advance, this question is very long, but I’m one of those that think the more thoroughly a situation is presented, the better it can be assessed.
You can skip to the end for a (very) short summary though…

We are a group of good friends, we’ve all known each other for 7+ years, since high school, and have been playing DnD since then, first 3.5 and then recently 5th edition.
One of the players, let’s call him John (who also DMs regularly, we will come back to this) has always been a good friend and constructive member in the various parties we have played.

I insist on the fact that John is not only a good friend, but also usually a very pleasant person to be around, and by no means troublesome, in most situations.
Our problem is, he also happens to be a very problematic player when you are the DM, and lately this has grown to such proportions, that it became somewhat toxic for the group.

John likes being cool. Here’s a list of John’s various “moments”, to explain what I mean by how problematic he can be for both the DM and the group

  • as a level 3 Rogue, spending 5 minutes trying to convince an NPC merchant to buy a broken wooden wheel for 10gp, before trying to sneak behind the counter while the merchant could still see him (no magic, no tricks, just… going around crouching trying a stealth roll), and of course getting caught trying to pick pocket the NPC, keeping the DM from interacting with other players the whole time.
  • with the same character, trying to steal from a treasure in the hold of the dwarven ship we were protecting, after multiple warnings from the DM on how both the magic protections and guards would make it hard for a confirmed burglar to pull it off, needless to say quite impossible for him (and of course, nowhere to run once he was caught, we’re in the middle of the sea).
  • stealing a horse in daylight in the middle of the biggest city in the realm, because the merchant didn’t want to lower his price, in front of said merchant, with guards all around this part of the city. Note that even if we weren’t pressured in any way, no thing that would force us to act quickly, we did on the other hand know that the king there would take the first opportunity he could get to have us locked away.

In most of those situations, when he was presented with consequences – that were honestly really moderate considering the foolishness of his character’s action, take the failed theft on the boat, we managed to convince the dwarves not to iron a “T” on his head for “thief” and had him simply imprisoned for the duration of the escort mission instead – he would get mad and sulk, taking any occasion he got to make it known that he was upset and holding grudges, which to be fair ends up ruining the mood for everybody in the party, not only the DM.

Since John grew more cocky as the quests went on – since we as a group would rather shrug his attempts off than wasting time having him be in jail, punished, and so on – he actually is the very reason the two other DMs (my co-DM included), have regretfully resorted to applying more logical punishments in their own campaigns, which isn’t really a problem for anyone, except John when he messes up.

I have read the very good topic about the My Guy Syndrome, and I believe this applies to John’s behavior very well.

As an example, I shall take his latest “feat”. He attacked an enemy camp we were just meant to scout, by himself, despite the warnings from the DM and attempts from the players to explain that it would most likely mean death and mess up the quest, saying “that’s what my character would do”.
The camp being occupied by approximately 40 to 50 well trained, armored soldiers, a lone Level 5 barbarian stood no chance, and so the DM talked openly with him about the consequences.

John was proposed with various scenarios, among which were the simple death of his character (and the introduction of his future new one in the story shortly after), or being captured, interrogated, etc… and then presumably rescued by us later, which was his choice.
It was also agreed that after suffering grave injuries as a result of both the battle and his imprisonment, his character would be severely weakened for the next session, which meant various debuffs, that John seemingly serenely agreed to.

Except the next session, John just showed up with the firm intention to sulk the whole time, playing on his portable device most of the time, arguing that his battle oriented character was impossible to fight with and hence could only do “boring stuff his character would never want to do”.
Note that his character has also refused every opportunity presented to him to be involved in roleplay while his imprisonment lasted during the previous session, which didn’t prevent John from openly complaining about how boring this part of the quest was for him.

For those of you that survived reading all that (congratulations!), you might be wondering : ” But why don’t you discuss this directly with John ? “

And this is the reason we’ve decided to ask for help, because what we thought would be the only logical way to address this issue has failed. We have tried talking about it with him, in a very diplomatic and honestly friendly way, not putting him on the spot, and his response is usually to lash out, as he experiences it, in his own words, “as a personal attack”.

This is where I have to bring up the fact that John also is a DM, for his own campaign, which btw is very enjoyable for everyone. But you would also think that this allows John to know what it is like to DM (he has for years now), and that it would help him understand how his behavior is problematic.

And you’d be right, John actually happens to be a reasonably strict DM, with high expectations for his players when it comes to both roleplay and commitment to the quest, and that is 100% fine for everyone involved.
What isn’t fine for our players, on the other hand, is to see someone so demanding as DM behave in ways he would never tolerate in his campaign, and berating others on their lack of roleplay, for instance.
Now, to be perfectly fair, we are a “happy go lucky” kind of group, and we all enjoy roleplaying as much as we like making jokes and comments “out of the game”, as players and not characters, which doesn’t sit well with John when he DMs, and apparently now annoys him as a player too.

What really pushed us to ask this question here, was that even though the last quest was still fun, John’s behavior is sometimes getting out of hand, and his constant sulking and post-game remarks are starting to be a real weight on the group ; his latest comment on how he “knows that DnD is not a passion and does not involve a significant investment from everyone” really is ticking us off, as all the players are really passionate and involved in both their characters and the campaign.

Just to be clear, we do not want him to play his characters in a certain way, this is not about coercing him into what we think is THE WAY to play, OUR way. As a group, we have always agreed on the fact that in the settings built by the DM, the PCs are free to do whatever they want, so that they might RP what they feel like, and feel free to find any solutions, any path, etc…
This is about destructive behavior that ruins the fun for everybody, using freedom of action as a pretext for “YOLO” actions.
And even though we are upset about this current situation, this is in no way a “vent our frustration” question ; we really feel out of options and seek for advice on how we should handle this situation

TL;DR : A party member regularly plays his character in such a way that endangers not only himself, but the whole group and sometimes even compromises the campaign.

He is aware of it, but also very defensive about it, he also turns into an insufferable grumpy kid whenever he has to face consequences, ruining the party’s mood, and gets aggressive when confronted about it.

We do not want to exclude him, as he remains a very good friend and we would rather solve this problem in a way that allows him to play, along with everyone.