Some of my players tend to forget that they're not their character when we play, which sometimes results in characters doing things they usually would never do. As the DM, I've been pretty lenient about this since they have been playing for just a year now, and still have much to learn.
However, it recently became a problem when the Lawful Good Deva suddenly decided to incapacitate his entire party and abandon his god simply to pick up a magic item that was undoubtedly of Evil Nature. To be specific, it was the Coat of Eyes, from the Khyber's Harvest adventure. This has caused many a conflict in the group as some of the other players think his alignment forbids him from doing that and he shouldn't have been allowed to. One player even asked me to have the Deva's god smite him for his wickedness, which I didn't do.
At the time, I wasn't quite sure how to respond to this so I changed his alignment to Evil, to which he strongly objected, stating that he merely wanted the artifact "so no one else could get it". Instead of destroying or hiding it though, he wore it. To me, it seemed like a thinly veiled attempt at justifying him grabbing the most valuable item so far (the coat is really good stat-wise).
What do you think? Should I punish characters if they irrationally behave the opposite of what it says on their character sheet? Can I, as the DM, even justify changing a character's alignment or should I keep out of it, as the behavior of characters is none of my concern? And most importantly: How hard should I bring the hammer down on that kind of behavior?