One thing that could be that necromancers that want to help the city guards, could have inquired who are willing to be animated. This could be indicated by a certain tattoo in an obvious place on the willful guards body.
They use arcane rituals to animate the skeletons of their fallen rather than necromancy so it's not evil and the skeletons won't go rogue.
As for necromancy being evil, the main reason is that it is the antithesis of life, an unnatural force in the world which is naturally going to be seen as evil by the average person in many campaign worlds.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.