Necromancy exists in our setting, and it is quite a threat to the living people of the world. Many past necromancers have left their scar upon the world, leaving certain areas as seemingly permanent wells of negative energy which have threatened society as a whole many times in the past. The existence of necromancy is an existential threat to the living.

Necromancy is soul-manipulation magic. Some necromantic spells are rather harmless, or even helpful, such as spells that destroy undead by separating the soul magic holding them together, allow you to speak to the souls of the dead, or manipulate your own soul. However, some spells and effects either actively kidnap souls or destroy them outright. These effects are considered to be utterly wrong, on a basic, visceral level. Most people, when confronted with the reality of such spells, would feel real revulsion for what is being described.

Necromancers by definition are not simply practitioners who are using harmless soul-manipulation magic. They are defined universally as those who specifically use harmful and taboo necromantic abilities. There is no such thing as a morally gray or good necromancer. They know they are wrong, but they choose, for whatever reasons, to pursue this practice in spite of the potential backlash.

Necromancers are shunned and feared in society as a whole. It is not a reasonable or acceptable decision to overlook their necromancy, any more than it would be acceptable to support cannibalism or blatant abuse in modern societies. Anyone who would willingly befriend or do business with a known necromancer would also be ostracized. No arguments made by either person would do anything to change how they are generally perceived. Actions speak louder than words.

Nilith is the primary deity of undeath, and she knowingly and willingly promotes necromancy as a sort of show of defiance towards the natural order, and more specifically, Lysara. Necromancers can find a certain amount of patronage working with the Nilithian Church, and Nilith even enables her strongest followers to essentially cheat death by becoming a willful (necropolitan) undead. There are some deities which allow necromancy to be practiced under their auspices. They either have no moral qualms about it or they find it actively useful for defeating their enemies. It should be noted, however, that their followers usually hide their disgusting activities from the public eye for fear of persecution.

Because Lysara is the judge of all souls, and because undeath undermines her authority directly, she is perhaps the most ardent opponent of the practice. Since every soul will be judged by Lysara, it stands to reason that most necromancers will find themselves subject directly to her wrath. This is why gaining a measure of immortality is so important to necromancers.

There aren't really any arguments that will convince most people that necromancy is anything other than vile, repulsive, an existential threat to society, and a fundamental usurping of the order of the universe. In fact, most who would make these arguments would generally be viewed as either insane or morally corrupt to the point of ostracization.