Wrap your mind around something has two meanings. To fully understand something; to accept or believe something as factual. In your sentence, the latter definition is more appropriate. There is something about the listener's past experiences with Preston that does not let them accept the fact that Preston is a killer.

The whole phrase is "wrap your mind around ____." It means "to understand." Example: "You can't wrap your mind around it." means "You can't understand it." "I tried to wrap my mind around it." means "I tried to understand it."