Originally I thought that Long Feng was controlling the king as a means of living the high life as resources could be diverted and for his own use. Upon seeing the latest episode, however, it seems that all he has managed to accomplish by hiding the war is a creating a considerably larger amount of work for himself than he otherwise would have.
By hiding the war he essentially has to fight on two fronts as he has to constantly monitor the goings on within Ba Sing Se and fight the war (or there woud not be a Ba Sing Se to monitor).
If he is working for the Fire Nation as a spy, why hasn't he advised the King to withdraw some troops from the defences in this time of 'peace' thus allowing the FN to easily invade.
If it is an ego trip then surely he must realise that by not concentrating the city's resources on the war effort, eventually they will run out of soldiers and the means to defend themselves?