Can sihr (witchcraft, magic) kill?

Yes, there is sihr that can kill. When discussing different forms of murder the scholars have mentioned that the person who kills another by means of a kind of sihr that usually kills should be executed (qisaas – retaliation), because he has killed by means of something that usually kills. Ibn Qudaamah said in al-Mughni (9/330): “The sixth kind: if he killed him by means of a kind of sihr that usually kills, then he has to be executed, because he has killed by means of something that usually kills, like killing by use of a knife. If he killed by means of something that does not usually kill or something that sometimes kills and sometimes does not, then he has to pay diyah (blood-money), but qisaas is not required, because he intended to do harm, and it is akin to beating someone with a stick (which may or may not kill).”

The ruling concerning the saahir (practitioner of sihr) in al-Mawsoo’ah al-Fiqhiyyah (24/267) says that in the case where he killed a person by means of his sihr… the majority say that killing by means of sihr may be done deliberately, in which case qisaas is required. According to the Maalikis, evidence or a confession is required. According to the Shaafi’is, if the person whom the practitioner of sihr killed was his equal , then qisaas is required if the killing was deliberate, which may be proven by a confession on the part of the saahir, such as his saying “I killed him by means of my sihr”, or “I killed him with this type of thing”, supported by the testimony of two reliable witnesses who have practised sihr in the past but have now repented, who can confirm that this kind of sihr usually kills. If it is a type that does not usually kill, then the matter is more akin to manslaughter.

And Allah knows best.