They were very cunning, in that respect, an operating sense of sequential logic, not usually a part of mental impairment.From the presser -- even after killing LF/CD, teens kept up the facade with family -- calling and updating them on there whereabouts, sharing that they were having auto trouble, etc.
Given the teens were in contact with family and no criminal records, no history of violence - they were not immediately named suspects, and consideration given that they also may have been victims.
They understood, by reasoning and factual input , how to do some actions that would result in other humans concluding a concocted scenario. They were not projecting a fantasy, they were setting a scene, an alibi, a ruse to offset any questions being raised. This is psychopathy, not mental illness.
They absolutely understood the difference between façade and reality, … but fantasy was a chosen route.. it wasn't what overtook them, uncontrollably. They chose fantasy as a relief from boredom.