Necromancy does exist 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 many times in the past threatened society as a whole. The existence of necromancy is an existential threat to the living.

What is necromancy? Generally speaking, it is soul-manipulation magic. Some necromantic spells are rather harmless, or even helpful, such as those that destroy undead by separating the soul magic holding them together, spells that allow you to speak to the dead souls, or spells that manipulate your own soul. However, some spells and effects either actively kidnap souls or destroy them outright, and those 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 are generally shunned and feared in society as a whole. If you wish for a real life parallel, let's just use child molesters as an example. Consider that in our modern society, that no matter how nice or friendly a child molester might seem, they would be almost universally hated once their deeds were discovered. Not only that, but anyone who would willingly befriend or do business with that person 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. With all of this said, there are some deities which allow necromancy to be practiced under their auspices. Either they have no moral qualms about it or they find it actively useful for defeating their enemies. It should be noted, however, that 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.

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 utter 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.