Learn English – What vivid verb should I use when someone “turns into” a zombie

Since you're asking in a psychological context, are you looking for one of:

Detachment (detached)

Dissociation (dissociated)

Depersonalization (depersonalized)

Emotional numbness (numb)

Based on the updated question, perhaps you're looking for:

Nihilism (nihilistic): Nihilism is often discussed in terms of extreme skepticism and relativism; for most of the 20th century it has been associated with the belief that life is meaningless. Existential nihilism begins with the notion that the world is without meaning or purpose. Given this circumstance, existence itself–all action, suffering, and feeling–is ultimately senseless and empty.

From the Encyclopedia of Philosophy article on Nihilism, though there are plenty of other choices (e.g. Wikipedia).