I ran into a situation like this with a player. The player would always go off and do his own thing. Often times it was just some random nonsense. It got out of hand and became an issue when trying to keep track of what the party was doing and what the "solo" player was doing. I ended up tossing him into jail a few times and even gave him a few serious a$$ beatings from the local guards. Still though he continued to do what he wanted outside of the group. 

I had a tale with his a couple of times about it but it still was an issue and it really started to cause issues with the other players by taking time away from the game itself. Now Im not opposed to splitting groups up and whatnot but this was out of hand. I think that for most of us the point of playing is to work together as a group. I really started to think he was just trolling the game because like I was saying, everything he did was just so random and had no reason or cause for it. He no longer is in the group. 

First off, please don't kill him. If I were him, I'd feel super cheated, especially because I was never warned that this could happen.

I suggest trying to talk with the player, like adults, or talk again if you already have. If you explain how the game is more fun for everyone (more "screentime," easier to run, etc) when the group sticks together, assuming he's not really immature, he should understand.

If he says it doesn't make sense with his "character concept," explain that the character concept is up to him as the player. He chose it, it's not immutable, and even now that the campaign has started, he can still change it, or add some realization, assignment, or even curse causing him to stick with the party. It might be as simple as safety in numbers.

If he says it's "forced," explain to him that, no, you're not forcing him to do anything, you just expect mature people to sacrifice a little of their own freedom so that everyone can have a lot more fun. If he still doesn't get it, don't confront his immaturity, just let the conversation rest and decide what to do next.

Because, if the player still doesn't get it, killing the character still won't solve the real problem, because a player who can't make some sacrifices for the good of a game with other players  is a problem regardless of what character he's playing. At that point, you need to think about whether this player is really someone you want to have in your group.

And if he wants to split up during a preplanned dungeon, well, then, don't intentionally  kill him...but let the dice have their due.

I wouldn't say outright just kill him, but you can arrange a scenario where if he just charges in alone, foolishly, he will die. If he doesn't stay with the party on his own and talking out of game doesn't work, make him *need* the party. 

Or just don't play with him. It sounds like the others don't particularly enjoy him anyways. 

Also, is it just me or does someone complaining about cooperation as "forced association" sound a little psychopathic to anyone else? 

That's just a really weird thing to complain about when you're sitting at a table with 3 other people on purpose, supposedly with the goal of playing a game together. 

I strongly believe killing the player is a viable option. Action economy is a bit deal. One player vs multiple enemies is a very dangerous thing. 

I have killed players when they were alone multiple times. In the end, each of the deaths were the players fault. They put themselves in the situation they found themselves in. We either have worlds that make sense or we run worlds filled with plot armor. 

Every game and group is different. At my table death is always an option. 

@CinciOmega, I totally agree, plot armor sucks, death is always an option...in fact, I killed characters in the last two sessions I DMed. I'm known for being ruthless, believe it or not! I just don't think the issue is one of players splitting the party by group decision--it's one potential problem player who's hurting other people's fun. That said, you're 100% right in general.