If someone commits a horrible sin as the result of black magic, is that person responsible for his actions?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

The bewitched person is not held accountable for the shortcomings in his/her acts of worship or the other sins he/she commits, provided the magic affecting him/her makes them act like insane persons, saying or doing what they do not mean or want to do.  In such a case the bewitched is like the insane, hence not held accountable for his/her acts.

Now, if the person afflicted with magic remains patient seeking reward from Allah, Allah will reward them and the greater the trial and patience, the greater the reward.  The Prophet (Sallallahu Alaihi wa Sallam) said: “Indeed, the greatness of the reward is subsequent to the greatness of the affliction.  When Allah loves some people, He afflicts them.  Now, those who are satisfied get satisfaction while those who are dissatisfied get displeasure.” [Al Tirmizi, al-Albani grades it as good]

Magic is the one in which, magicians use talismans, spells, charms and knots to exploit Satans and Jinns to influence the hearts, minds and bodies of people.  It is forbidden and is considered a major sin.

Allah knows best.