you see, unlike most people, I can understand that sometimes interparty conflicts can be fun when done right. But usually, this is done in the context that the bigger story arc at large is not really that important and all the other players are onboard for this with their own host of agendas. This also breaks down the entire "party adventure" model that D&D was designed for.

So, fundamentally, you need to let the player know what kind of game you guys are playing.