I guess you have never watched a family member descend into meth abuse then.

Permanent personality changes such that trust is pointless, redemption is merely a faint hope, all motivations are underlaid with a new narcissistic set of delusions and denials.

Since I have watched this happen to three family members of distinct lineage and personality types, all of whom were pleasant and acceptably kind beforehand, my small sample size is nevertheless very convincing.