He plays the armorer, which means he's tanky as fuck and has great saving throws. It also means that he plays as if his character is immortal. They're level 10, so anytime they see a generic group of humanoids, he knows that he can just get away with anything. Recently he went into a combat solo against one of the lead bad guys and six of his goons. The fucker managed to escape using Hypnotic Pattern on the boss, and wanted to turn around to fight his thugs on 1hp. With healing and my refusal to cheese rolls he managed to not die and decided to run back to the boss to try to steal his weapon. I wasn't about to allow that and he ended up just escaping. This was all in the middle of a city full of soldiers searching for them by the way. I'll blame my inexperience as a DM for this as there are lots of way that I should've killed his character in that instance. Everyone was perfectly polite during the whole thing too, but when the other players played out what they did during all this? He constantly piped up and had things to say. He even backseated them in character thanks to telepathy.
You can use minions/summons for the "Help" action according to the 5e rules, but he seems to think that means he gets advantage on any ability check as long as his homunculus is nearby. He's tried to use it to help him with climbing something and got pissy when I explained that a tiny creature with 4 strength can't help someone in plate climb anything. Even without the homunculus, he always tries to tack his character onto anything anyone else is doing in an attempt to "Help". He'll say he's going to help and then spend 30 seconds fumbling to figure out what that means when I ask.
I've tried to explain to him that turns are six seconds ingame time and they wouldn't be able to stop and have lengthy strategy meetings but he never shuts the fuck up. During battle he constantly tries to backseat the other players. I asked them about this and they don't seem as annoyed as I am, which is ridiculous to me. It completely takes me out of the scene and turns the game into a turnbased strategy. But then when I want to have the bad guy talk to them in the midst of combat that's when he starts questioning the length of turns.
In our latest session I had given them a magic container that heals them when touched for a full minute, but causes a random magic effect when opened. It was also cursed to charm whoever uses it and force them to open it. Our warlock went down and suggested using it. They'd only just acquired it and didn't know about the curse, I didn't remember exactly how it worked either so I said yes. I realized it says one minute after they'd healed, but figured it might spice up an otherwise boring fight. Well the item healed him and the random effect it caused was casting harm on everyone nearby. The artificer was downed and started telling me it was a shitty thing to do as dm, that I bent the rules to force a loss, etc. We had to pause the combat there due to time, but I'm extremely tempted to kill the motherfucker next time even if it means the group is done as a result.
Early on in our games we had another player in the party. To be honest this guy was kind of retarded and had anger issues, but I appreciated that he actually played his character and would suggest doing unorthodox things to solve problems. He clashed with our artificer though and ended up quitting. If there's one thing I regret, it's trying to appease them both and not just telling our artificer he sucks.
I guess I'm just fuming over being called a poor DM, which I don't even deny. I'm awkward and can't always come up with quick reactions for NPCs. However I'm also the only one of this group that was willing to take the time to do it.