RPG Descriptions are Important

Just read an article about old school D&D having trouble because Assassin and Thief characters would kill or steal from party members. To me that's a failure in GMing and in party cohesion. Sure an assassin is going to kill people, it's in the name. But they aren't crazed murderers killing random or just nearby people. They kill for money or mission. If the GM gives the assassin character a mission to kill another PC that's on the GM for hurting the game. In a very rare instance a GM may try to divide the loyalty or put a character in a position to betray the party. Think of Firefly,  Jayne nearly betrays Mal in the episode with the Fed or when Jayne does betray Mal in the hospital job. But I'd consider that some advanced storytelling that should be limited to players who value the group more than XP or "Just playing my character," excuses. The GM and player of the possible betrayer should monitor the health of the group and be ready to ditch the idea.

Thieves stealing from party members is a similar problem. A thief character should have ample opportunity to steal (and use other roguish talents) in the world without taking from their comrades. Just as an assassin isn't a psycho killer, a thief isn't a kleptomaniac. The thief character should value the party's support more than trinkets and baubles they can get from their purses. A player who ignores this should be addressed by the GM or the group. The GM can reinforce this with "Honor among thieves," and other setting fluff. For example a team of bank robbers have no problem killing cops and stealing from the bank. But wouldn't dare take a dollar more than their share, and would feel bad if they killed a bystander. (See Reservoir Dogs)

The article ends with a different example, in early D&D Barbarians had a rule that awarded them experience for destroying magic items and a class description that they hate magic in all forms. This is game structure that directly opposes Barbarians with wizards, clerics, characters who have leveled into spell-like abilities and everyone else who's carrying a magic weapon or item. Unlike the previous examples a Barbarian character of any alignment would be in character to betray magic using comrades, steal party members expensive and rare magic items and destroy them. Played with restraint and respect this could be role-playing tension. But as a general rule it justifies actions that wreck the game for everyone. Here the game was the problem not the GM.

I feel that the problem is partly one of growing pains, RPGs were changing from wargaming and "murder hobo" hack n' slash to the more storytelling games we have now.  Making these changes reinforced storytelling and was probably the right thing to do in the long run. Now we have a better base to build on. GMs and players are used to diverse parties and are less likely to use their class (or alignment) as justification to damage the game.