Black Rose is seen as such a terrible thing because of the lasting effects it has on the areas it is used in. Aside from destroying everyone that comes into contact with it, it also destroys the very ability of the land to support life. Any plant life hardy enough to survive its diluted form becomes literal poison that drives living things insane (something I only learned about recently when I finally went through the side quests to unlock the Ananta beast tribe) when consumed.

Sure we may have magic that causes a person's lungs to cease functioning, we can call down meteors that could cause massive craters and large amounts of devastation... but at least the land survives that and can recover to support life.

Not to mention, those spells we use to kill people are things we use at such a range where we have to face the fact we are killing people. A weapon like Black Rose is one such that it can be deployed out of sight, out of mind, and have no effect on the mental state of the person deploying it against their enemies. It allows them to easily cease to see the people they kill as people, and instead as numbers on an after action report.

Much like why chemical weapons in our wars were largely banned after WWI. The artillery crew would fire normal rounds to force people into the trenches, then fire chemical rounds to flood those trenches with toxic gas. To the artillery crew all they are firing is at a set of coordinates. To the commander ordering the barrage all that is happening is a set of trenches being bombarded.