I think this card should be in darkness because traitors are looked upon often as evil or bad. Also, a befriended creature turning upon its owner kind of reminds me of forces acting upon it.
Darkness isn't evil, never will be evil, and never has been evil. Get it right, or Bloodshadow (http://elementscommunity.org/forum/index.php/topic,22130.msg304589#msg304589) will rip you a new one.
Darkness is ideal, however, because traitors traditionally betray their allies on acts of self-preservation, which is the true nature of Darkness. Self-preservation provokes any kind of action, in almost all cases.
Well I should be either darkness or entropy. Darkness for the reasons you just said and entropy because well, you force a creature to disobey it's owner and to attack him. It's like antimatter only with attacking, if you know what I mean.
And yet, Entropy either deals with the laws of thermodynamics, or randomness. Are you being random? No? It's Darkness.
ERHEM.... also Disorder
hence Anti-matter and "CHAOS" seed.
I reckon it should be an ability similar to that of Maxwells Demon.
Heck! it could be a Darkness creature with an ability that costs 3
and even be a rare! (like Arctic Squid)
If the creature's attack is greater than its defense, cast "Traitor" on the creature.
(not being serious with that one, its just an example, be creative with it)