You're probably correct, but I sense something very cold and dark in Brenda's eyes, 'way beyond ditz-hood. Nothing would surprise me.
Here's a What If: What if Brenda had been not only sexually abused as a child, but also had been used by ritual abuse cult, possibly satanic. She then was hypnotized to forget, consciously, but unconsciously she was driven to re-enact her abuse as she grew up, eventually as the perpetrator as well as victim....This is X-Files tinfoil speculation, but her eyes are THAT cold and her behavior extremely bizarre. Did Brenda and her wimpy slave of a husband perpetrate sexual abuse on their own child? Did it culminate in ritual murder? Will we ever know the whole truth? Were all of Danielle's body parts found? Should I be writing fiction?
Or perhaps, as you infer, a local gang found Danielle, spotted her previously as a psychologically unprotected child, selected her as victim for a ritual, cultlike slaying to be carried out by member as initiation into the gang. Lots of brutal Mexican gangs prowl San Diego-LA area. Every morning, corpses of ritually abused animals are found, and sometimes human corpses. The police may recognize some clues in the way Danielle's body was disposed of. So many people knew of the VDs habits, almost anyone could have carried out abduction of Danielle when they knew her loving mom and dad were otherwise engaged.
It's so easy to come up with the "what ifs" in this case,it's hard for me to accept Westshisname as the murderer. Seems too pat.