Using jinn or witchcraft to harm people is a major sin and mostly amounts to disbelief. The effect of magic cannot be deniable. The Qur’an and Sunnah are full of evidences on this fact. Allah says, “And they followed what the Shaitans chanted of sorcery in the reign of Sulaiman, and Sulaiman was not an unbeliever, but the Shaitans disbelieved, they taught men sorcery and that was sent down to the two angels at Babel, Harut and Marut, yet these two taught no man until they had said, “Surely we are only a trial, therefore do not be a disbeliever.” Even then men learned from these two, magic by which they might cause a separation between a man and his wife; and they cannot hurt with it any one except with Allah’s permission, and they learned what harmed them and did not profit them, and certainly they know that he who bought it should have no share of good in the hereafter and evil was the price for which they sold their souls, had they but known this.” (Al-Baqarah 2:102 )‏

In this regard, we may quote Sheikh `Atiyyah Saqr, former head of Al-Azhar Fatwa Committee, who stated,

The word “magic” is mentioned in the Qur’an 60 times. The verse 102 of suratAl-Baqarah has the following implications:
1- Magic has been known as a well-established fact regardless of it changing the forms of things.
2- Magic involves things that may be beneficial or harmful, as Allah says: “learned from these two, magic by which they might cause a separation between a man and his wife…”
3- Magic cannot have any effect save by the Will of Almighty Allah, as Allah says: “…and they cannot hurt with it any one except with Allah’s permission…”
4- Adhering to magic renders one a disbeliever, as Allah says: “…but the Shaitans disbelieved, they taught men sorcery…”
the views of Muslim scholars carry culminate in the following:
1-Attributing the effect of the magic to anything other than Allah renders one a disbeliever.
2-Practicing magic or sorcery for the sake of harming people is totally forbidden. Based on this, it is a form of disbelief to torture people with a magic spell or subject them to the evil of jinn.

As for harming people with magic after their death, this is simply absurd and an inconceivable lie. Life after death is beyond our influence.

Similarly, the claim of using angels to hurt people is more absurd. This claim even contradicts the clear Qur’anic verses that describe the angels as, “Honored servants. They do not precede Him in speech and (only) according to His commandment do they act.” [Al-Anbia’ 21:27-28] and that “They do not disobey Allah in what He commands them, and do as they are commanded” [Al-Tahreem 66:6].