Something worse than undead

[Oberon Kenobi]

Man is the real monster

Cliche though it might be, this.

Point to the wars, the barbarian hordes, the bandits and plagues and whatever else that turned man, woman and child into corpses in the first place. That's your real evil. The necromancers? What they do might make people uncomfortable, but the undead they raise can help rebuild a land ravaged by war and guard against far worse things. Like keen steel and keener malice.

Scapegoats only hold the people's attention until the threat is gone. If you really want them to stay on your side, you need to convince them that you're on theirs instead of just being the lesser of two evils.

...Mind you, if the necromancers are actually evil and are just lulling the kingdom into a false sense of security, then yeah, demons and mind flayers are great.