How do I deal with my former DM turned problem player? [closed]

Question

I'm currently 9 sessions in to a brand new 5e homebrew campaign, with a greatly varied party of six players; two newbies, two intermediate players, one very chill wringer veteran, and my previous Pathfinder DM, who is about as new to the 5e system as I am. He's DM'ed for me in three previous short term campaigns, and he's a decent DM with a tendency towards railroading. This is also my first time DMing.

I've been having trouble establishing a good rapport with him as a player in this reversal of roles. He has a tendency to rules-lawyer, which I've tried to call him out on subtly, but the biggest issue is how he's treating my other players.

He's playing a NE rogue arcane trickster who is secretive, manipulative and likes to split off from the party if they cant solve a problem by themselves. This leads to very overt sighs and grumbles when I'm doing a scene with the rest of my players. I'm trying to make the world a good balance between sandbox and storyline, but this player is trying to shotgun the story (i.e. jumping headfirst into deep story progression without taking time to explore or let other players catch up), which is making my new players, who are still exploring the mechanics of the game, kind of lost and uncomfortable.

When he's not doing that, he's bullying other players into making skill checks to gain knowledge that he wouldn't have previously had, and just generally gives off a sense of displeasure that he can't get away with lawless mayhem and thievery, even if he fails the appropriate checks I ask of him, and seems annoyed when my NPCs become realistically distrustful and hostile.

I don't want to kick him from the group; we've been friends for years, and I genuinely do like his character's potential, but I greatly dislike feeling undermined in my authority as a DM, and the feeling that my other players' fun is being stepped on. I need to to talk to him about this, so do y'all have any advice on what I should bring up as suggestions for a change in behavior? Because I don't think in-game subtlety is going to hack it for much longer.

Answer 1

2 Things:

1. Talk to him out of game and explain to him what you did explain to us. This is the first thing you should be doing. He is an experienced GM, so I expect him to understand your position and the work you are putting in this campaign.

2. Run his antagonism vs. your group fairly and don't force your group to metagame on his behalf. If he runs off to do stuff on his own, give him 5 minutes in real time to do so. Then, give the rest of the group 5 minutes for each player in the remaining group, so e.g. 15 minutes playtime for three other players. If he complains, sighs or otherwise passive-aggressively tries to signal his displeasure, explain what you did right there. Address his behaviour directly and on the spot.

"Dude, you got 5 minutes of solo play, now I have to be fair and give the other players 5 minutes each, don't you think?"

Make it clear that your group does not have to entertain a hostile, evil PC just because the character is played by this player. Allow them to kick the aggressive character out, or leave him behind. If they do, have the player make a new character who plays nicer. This is nothing more than the other players playing their characters. No one wants to be in life-and-death-situations with someone who can't be trusted or who you suspect works against you. Communicate this clearly to them.

Usually, players in this kind of situation want to be polite and not cause conflict. Tell them it's ok to not humor PCs who are a detriment to the group.