Would we call Ben Gurion a traitor because he left MAPAI and formed RAFI? I doubt so. If a politician, a statesman feels that his party's line is no more consistent with his own views, he has the right to leave it and join another party, more suitable for him, or form a new one. It is exactly what happened with Kadima.
Labor court orders Jerusalem light rail operators to return to work (Haaretz)
from the article: Katsav to summon Olmert on Thursday to form government