Musar zu Bamidbar 13:26

A most puzzling aspect is the fact that all these twelve men at the time when they commenced their mission were obviously suitable, highly thought of, else Moses would not have selected them. What happened to change the outlook of these people? When the Torah in 13,26 states וילכו, ויבאו, this is to tell us that just as they were corrupt when they returned, they had already been corrupt when they set out. How is that possible? We must conclude that they were indeed wicked to start with, but since they occupied positions which the Torah has described as ראשי בני ישראל, leaders among the people (12,3), they were presumed to be righteous, since Moses did not have the ability to look into their hearts.