Praise be to Allah, and peace and blessings be upon the Messenger of Allah. To proceed:
The term zombie is used in modern times to refer to the living dead in horror films, stories, and works of fiction. There is nothing in Islamic creed that establishes the existence of this kind of dead person who dies and then repeatedly returns to this worldly life.
The basic principle is that when a person dies, he enters the world of Barzakh, and does not return to this world until Allah resurrects him on the Day of Resurrection. Allah, the Exalted, says:
﴿حَتَّىٰ إِذَا جَاءَ أَحَدَهُمُ الْمَوْتُ قَالَ رَبِّ ارْجِعُونِ لَعَلِّي أَعْمَلُ صَالِحًا فِيمَا تَرَكْتُ كَلَّا إِنَّهَا كَلِمَةٌ هُوَ قَائِلُهَا وَمِن وَرَائِهِم بَرْزَخٌ إِلَىٰ يَوْمِ يُبْعَثُونَ﴾ [Al-Mu’minun: 99–100].
“When death comes to one of them, he says: ‘My Lord, send me back, so that I may do righteous deeds in what I left behind.’ Never! It is only a word he says, and behind them is a barrier (Barzakh) until the Day they are resurrected.”
Extraordinary events and miracles may occur through which Allah brings some of the dead back to life, as happened during the time of Jesus (peace be upon him). However, this is an exception and a miracle; it is not a general return of the dead to worldly life.
Accordingly, believing that the dead return to this world as zombies as a real phenomenon, in a manner that contradicts what is established in Islamic creed concerning the deceased remaining in Barzakh until the Day of Resurrection, is a false belief that contradicts the revealed texts.
As for merely watching zombie films, using the word “zombie” metaphorically, or talking about zombies as fiction and stories that have no basis in reality, this is not disbelief. Such things do not involve believing that zombies actually exist.
However, if someone believes that the dead continuously return to this world, or that a person dies and then returns in another body, in a manner that entails denying the Resurrection, the gathering, and the reckoning, then this is disbelief because it entails rejecting what has been established in the Qur’an and Sunnah concerning resurrection after death.
In conclusion: Zombies, in the fictional sense commonly portrayed in films, are not a reality established by Islamic creed. A Muslim does not become a disbeliever merely by imagining such stories or watching them, as long as he does not believe that they are real. However, believing that the dead return to this world in a way that denies the Barzakh, the Resurrection, and the reckoning is a disbelief-based belief because it contradicts a fundamental principle of faith.
And Allah knows best.