I think that there are 2 basic forms that one can learn.
"good" necromancy- the honoring and humbly asking thing of the dead
"bad" necromancy - the summoning and forcing the dead to do things for you.
There is also what is known as "vampiric Necromancy", where one consumes the life energy of another to replace depleted essence of their own. Some Necromancers use knowledge of the soul to manipulate the living. Others simply work with the essence of Death itself and may travel to the realm of Death.
Others may also attempt to make themselves able to withstand deeper immersion in the essence of Death, to better wield its ability and to increase understanding. To transcend flesh is this goal.
And yet again there is the type of Divination done with the dead, or medium work (which could be considered a form of Necromancy).
I would say, that with any magic, "good" and "bad" is simply in how one uses their talent and how society percieves this use.
There is no certain morality in necromancy and in its practice,tho there is ignorance and ego which is consider filth.Opposite of it is emptiness which is honored.
The dead don't stay forever in underworld.One second last almost eternity in underworld.Tho its not right to drag dead from underworld,however its not nice to drag anything to serve as tool of someones will.
Death as a universal primal form is very old.Originating from light khaos with touch of good old primal empty KHAOS.Thus necromancy can be surprisingly wide and large subject,not some simply energy manipulation of energy and calling of death related entities.
Forcing dead or playing with death energies is very harmful to almost everyone.Not in the moment tho.
It takes a unique mind frame and soul to devout to necromancy.