From my experiance, I would not say that necromancy is what has been mentioned here.
A popular idea of necromancy is that it is either Just summoning spirits (usually for divinatory reasons) or creating Undead monstrosities under ones command in one way or another.
I would like to express that both ideas are false.
Like other forms of magic, necromancy is Not simply one sided.
There are many ways to practice such magic, and it doesn't Have to be limited to one specific method and result.
There are forms of necromancy that: study the nature of the soul, lay and remove curses (not specificly killing in nature, though withering and diseases are closer to common), cause change, create serviators of death energy, speak with the dead (not necessarily Only for divination), cast spells with death essence, astral projection into areas that have concentrations of either death entities or spirits of the dead, mastering ones emotional states and helping others with the grief of losing loved ones, communing with death related entities, and making talisman/fetish/totems of death energy.
Those are things I would consider Necromancy, not grave defilement, ressurection, or indescriminate killing through magic.
The necronomicon was mentioned, and while I have heard of people using variants of its teachings to achieve their own pathworkings I wouldn't say that it is specificly necromancy in itself.
Necromancy is one of the paths of magic that you do have to sacrifice to study, as comming closer to death can and will effect ones appearance and prehaps overall health. Mediums who channel too much too often can take in some of the energies of the dead and become ill, this effect is much more concentrated in the body/spirit of a person whom specificly seeks out deaths energy.
^ is a website created by a man named Tomekeeper, outlining points of his variant of necromancy (Direct magic, ie a lack of rituals) and also presents his point of view on older necromantic tomes and grimoires, which he cites his research on and reasons for critiquing.