Are Graff and Mazer justified in their deceit and manipulation of Ender? why or why not?

I would say not especially considering the Buggers were not the enemy Graff had  thought they were. Gaff and Mazer were both blinded by their military training and experience. They used Ender as a means to an end. They neglected that Ender, and the others, were still children. They let Ender slaughter a near entire species without considering Ender's emotional fallout.