It may come across as a simple thing but hallucinations are often at the root cause of major problems people have that cause them to commit horrific acts on others. While just having those may not be common, as it is often associated with something more, it is quite clear that they are an issue. In the past, hallucinations led to some Kings to declare war on people and made certain military leaders commit mass murders.
If we’re being honest with that knowledge, we cannot overlook hallucinations as a major cause to people being part of a real zombie apocalypse even if they do not know about it. The Marvel comic book villain Mysterio uses this to force people to do horrific things, even influencing Wolverine to assume the X-Men were enemies. Thus leading him to kill everyone in sight. That said, the power of hallucination could be used in a similar fashion by a real person too.