Theft is (generally) an evil act, IMO, and certainly evil when you're stealing (out of selfishness) something from a teammate or good-aligned individual.
That's not to say that a rogue picking locks for the party is evil, nor is he evil for lifting the key (and purse) of the dreadful prison warden in the adventure, nor is he really evil for stealing from evil NPCs, nor is he evil for stealing a dangerous artifact or item from a good NPC, etc. But stealing from a teammate to satiate one's own avariciousness and ego is an expletive Eric's grandma wouldn't like, and it's evil.
I would disagree here. Theft is a certainly not a
Lawful act, but it is not necessarily an Evil act. Even in a society of evil creatures stealing is against the "law", against the "order", however tenuous, of the society.
Making Theft, in 9-point Alignment terms, a non-Lawful act. Now,
how non-Lawful must be examined on a case-by-case basis. Stealing from a party-member, I would say, could go either way.
Was the barbarian just being selfish, he just wanted that item for himself? That is completely within the realm of CN (or NE and possibly, in some specific situations, even get away with it as a True N or NG). Making the theft a Chaotic (against the "order" or "rules" of fair/even distribution) act with Neutral intentions (just selfish, not looking to benefit-"good" or malign-"evil" anyone else. Just out for himself) .
OR was he being purposely malicious in his selfishness, purposely desiring to created dissent and inflict misery on his fellow teammates, which I would attribute to more in line with an "Evil" (the unconcerned harming of others) bent. In this case, the theft isn't really the point, it isn't Lawful, but it really isn't Chaotic either. It is simply the mode for "getting at" one or more of the party members. So a Neutral act but with decidedly Evil intentions.
A CN character who is overtly "evil" in their actions, runs the risk of alignment shift/change, as are characters of any alignment,
Not necessarily/automatically to Chaotic Evil, but to Neutral Evil first...then if the acts continue to be chaotic and evil, then the next Alignment slide is from NE to CE.
At least they would in my game. Unless, of course, the action was so heinous and disturbing (strutting into the Jedi temple and slaying all of the student children in cold blood, for example) that they would just vault over/beyond the pale of NE and straight into CE.
This case with the Barbarian does not seem to be such a case and does sound more like the player wanting to screw with your head than any serious-play alignment-based actions.
IN CLOSING (I know, this was long), I would submit to you that you submit to your DM a written out set of guidelines, not hard and strict "rules" mind you, but a well thought out set of Alignment definitions. All of the players get them. Everyone's on the same page and can't pull the "but I was acting in alignment" card out to justify whatever they want...at least in many cases. I highly recommend the definitions set forthe by cperkins in the pdf link in my previous post. Short, sweet, and covers all of the main "points" of each alignment. They'd be a great starting point for you, I think, and saves the DM having to examine and philosophize and come up with everything on his own.
Good luck with the new group. I think you'll have much more enjoyable experiences without the jerk. But do understand, getting along with (or at least tolerating) different game styles, personalities and personalized definitions/rationale for actions and alignments is one of the...let's say "joys" of table top (or really any) RPGing.
--SD