The undead aren't strictly alive, however. They're just magic automatons, mobile in spite of themselves. And there's no reason death magic can't be about both smiting people and using their unnaturally animated cadavers to kill even more.
Well, they are more alive than they were before.
Actually it does not matter whether the regular undead are in fact mere automatons, the necromancers that control them are (in present DF) themselves undead are are quite sapient. The zombies were dead before so they did not lose anything by being raised as mindless automatons while the necromancers are now freed from having to make their own writing materials so they can spend forever writing books recording their centuries worth of wisdom for posterity.
The point is not that folks could not use the magic *of* death to raise zombies, it is that from a morality POV a civilization that holds death in high esteem would likely find necromancy appalling even it used the power *of* death to thwart the process of death. Using death magic to kill folks however is however quite in alignment with the value of death since you are spreading death in the world by killing more folks.
There is a potential distinction here between the value of death and the power of death. If I hate death but have the ability to become immortal only by using the power of death to make myself undead then I will do so if possible. If I love death then I will not make myself undead even if the power of death allows me to do so because I want to actually become fully dead one day. If magic is tied to the value of the thing the power of which you are wielding, so that in order to wield death magic you have to value death, a situation occurs that while death magic may be used to raise undead nobody who has the ability to do so would ever do it.
Logically however, necromancy could equally be an expression of the power of life (so the name stops making sense)
. This is actually a far worse situation because those who value life are more liable to hate death, so there is a quite logical path for someone to use the power of life to make themselves immortal through undeath rather than accept that their time is finally up.
Necromancers really get a bad rap, considering all they really want is to raise a family.
On another note, is a group of necromancers called a "corps" of necromancers?
I am not sure it is not because of the sheer potential threat of the zombie apocalypse. The more you kill, the more zombies you can raise and the more zombies you can raise the more you can kill. The powers that be hold to the inherent evilness of necromancy because they must stamp it out wherever it gets established, since the only way the zombie apocalypse can be stopped is if you can deploy overwhelming force before it gets going. This means necromancy must be considered the ultimate evil or else folks would not put aside whatever other disputes they have in order to unite to assemble such an overwhelming force. There is also another problem with necromancers from the POV of the living.
Once sapient undeath exists life is now obsolete.