You are in a no-win situation. This person has issues that go far beyond the game, and how it turns out basically hinges on whether he can recognize the issues and address them. Do not, under any circumstances, feel guilty about what you have to do to keep your group functional, even if it means booting him out. And if you feel that the best thing is to accede to his wishes and let him play what he wants, be prepared for more manipulation down the road.
This. First and foremost you should be concerned about group health. A good social atmosphere makes for a good gaming atmosphere. Losing one player who is already grinding with the others is much better than losing a group to please one player.
Try offering to let him play a new character, but not one that overlaps so heavily with another player's? That way you might be able to avoid some of the competitiveness while he can switch away from a character he apparently didn't like?
Assuming the OP wants to keep the player in the game, this is probably the best course of action. There are plenty of awesome min-max builds out there where a "diva" player can shine, without taking over the nice of another player.
1) ask the rogue player if he/she wants to change and, if so, let the Player in questioin play his assassin-rogue
(as a warning from personal experience, the personality type you describe combined with wanting to play a rogue-assassin would set off a red flag in my head. that type of combination in the past has usually led to a PC that is as disruptive as the player: the pc will want to run off and do things solo, all the time leading to everyone else sitting around half the session; or he'll try and murder people for no real reason, etc etc. that's not to say this player will do the same.just saying that's been my experience with the combination of player and character archetype that you're describing)
Getting both players to discuss this could also relieve some of the issues. If the current rogue player is perhaps also unhappy with his character or has no issue, then it's a non-issue IMO. But you never know, people react different when talking to a person in a leadership position vs people who aren't. You might find all your players favor a whole new party makeup rather than figuring things out via DM.
HI all
Basically he seems to like optimising characters, so he wanted to swap out his Warlock, I said see if you can keep him but change his powers/pact etc. This seemed to delay things, but increasingly he'd been acting at odds with the party, provoking the defenders etc until he got this warlock killed by attacking another PC so he could now play this new character (risking another PC to do it).
My group has a similar player, though it's been less of an issue, no matter which character he plays, he is the party diva. The party backs him up, or it starts a fight.
So I retconned this as I wouldn't have allowed this to happen if I knew what he was up to (long story). So I said no to his new character an Assassin Rogue Hybrid, partly because the party had a Rogue and needed a controller and because I think he is doing it to outshine the Rogue and play competitively again.
No offense, but retconing whole situations is generally a bad idea. Comic books can't do it, small games can't do it. Likewise, as it's been said several time, don't force party composition. Especially if you're looking for a fairly niche role. All classes have some controller effects. Defender marks are supposed to control, striker effects are supposed to control, healer effects can control as well. "Control" is also an issue of tactics, if your NPCs are not being controlled, then perhaps your party needs to work on it's tactics better, or perhaps you the DM are ignoring effects placed on the NPCs when they shouldn't be ignored.
A lot of control can come from simple pushing and slowing. You don't need a character MADE for a controller, because especially at higher levels, all classes have so much versatility, a straight controller loses a lot of his niche.
Anyway having said no, he still showed up with the character and began to play it until I realised he wasn't playing his previous character (again long story here). I told him he had to play his old character or role up someone, anyone else as long as it wasn't a Rogue. But he refused and told me he was going to play his new character and that was that! So I said enough, you can sit this out and we'd sort it out after the game. I talked to him after the game briefly and took his character to see what he'd done. But in truth I don't feel he should play this character for the trouble it will cause and the way he tried to engineer it to.
In short, this is your main issue right here. He's trying to play boss and he isn't. For all the player generated content arguments I've had in other threads, when a game is clearly set up that the DM is in charge, THE DM IS IN CHARGE. End of story. He needs to either understand that, or leave.
The thing is as I've got to know him I've found out he is desperate to DM, but the rest don't appear to want to be his players. Secondly he is very competitive and can be childish when he doesn't get his way.
Then it's no surprise that they don't want to play with him. Do not, at all costs, let him DM or feel that he has some control over the game or it will start costing you other players.
I'm looking over his character trying to decide if its a good idea to let this character roll, or stand my ground and potentially loose him from the group. I think the thing is I could be misjudging him, but the rest of the group was embarrassed by his behaviour. Who needs that in a game which supposed to be fun?
If you can come to a compromise, do so. If not, give him the boot, as he sounds incredibly detrimental to your game.
As a final note, I don't know if you're alloing Essentials or not, but I would point him towards the Hexblade. It's a melee striker that's basically a warlock-rogue hybrid. It may prove different enough from your other rogue to warrant working in the group, but at the end of the day, what seems to be working the least is the player, not his character.