Lord Crimson
Explorer
I do not think that everyone in the party has to be " Lawful " Evil, but they do have to be " Loyal " Evil. If there is nothing uniting the party except the pursuit of profit, then there will be nothing stopping anyone from killing their fellows if it would profit them.
You could think of Aristotle's types of friendship as well. If they are friends because of profit it will not work, if they are friends because they enjoy each others company it can work (as long as they enjoy each others company), and if they are friends because they have some sort of mutual respect and desire for well-being, it will probably work.
This has been my experience. Playing a party of evil characters =/= playing a bunch of PCs that spend all day screwing each other over.
I had an evil campaign I DM'd for years. All the players did was operate from the assumption that dicking over the other PCs was just not acceptable.
Dicking over anyone else, though? Totally fair game.
Hell, I even had them working for more heroic NPCs. Because, when the chips were down, they were the most capable people in the region. And knowing that things were bad enough that the "good guys" were coming to them? Well, now they could justify an extortionate "reward" for their "good deed".
And even evil characters know that something more powerful and more evil than them is dangerous and needs to be stopped. And if it threatens their lives and illicitly-acquired goods, castles, and nations? Now they're really motivated. ("We stole all this stuff fair and square. No way we're letting you steal it from us.")