Stealing souls is not a good idea, even for the most advanced magician who can succeed in doing so. It sounds surreal and fictional, but it is technically possible even though the magicians doing it should know better. The issue with stealing souls is not that a soul is destroyed because that is impossible, but that soul stays with you. If you consume a person’s soul, you must keep it and it cannot be returned or separated from you so easily. The soul will fuse with yours, and it will even compete for control of your physical body. This can appear to outsiders as Dissociative Identity Disorder but there will not be a protective logic, the other person’s soul can mess up your life from the inside, and even a brief loss of control by the other person’s soul is enough for them to put your hand on a hot stove or leap into traffic.
As the other person’s soul tries to fight yours for control of your own body, you may begin to think you actually have DID. You may find items in your home you don’t remember buying, be accused of saying things you did not say, or may end up in trouble for things you would never think about doing. The difference is that the soul stealer’s curse is consistent and dissociation is a response to stress or traumatic events. DID is understood as soul fragmentation where trauma has caused a person’s soul to break and parts to act independently, but the soul stealer’s curse is caused by the reverse problem where two souls compete for a body as one tries to escape and wants to cause the body’s physical death so it can leave. This is the stuff of horror for both parties involved. Even if it cannot cause issues directly, its energy can radiate into the captor and cause it some very extreme misfortune. Even attempting this kind of magic is asking for a catastrophe.