It's like this: Nekron is wearing these bodies, AND their brains and memories, as puppets. Think of it like he dug someone up and wore their face or something grotesque like that. He's taunting, using guilt as a button to push to stimulate other emotions. The perception that the dead are blaming you or reproaching you, dead who are emotionally important to you(leaving aside the villains, who mainly seem to be focused on the lowest common denominator, fear), and are convincing because of their appearance and the specifics of what they say, will be more likely to overwhelm your rationality and let your emotions get away from you, including anger that the corpse is being used that way and desecrated.