Agreed, if as Harris (defense attorney) has suggested, DB is schizophrenic, we may never know or understand motive. I don't want to try to diagnose DB, but I do believe he suffers from serious mental health issues. If he does suffer from schizophrenia he may be subject to some or all of the following symptoms:
Behavioral: social isolation, disorganized behavior, aggression, agitation, compulsive behavior, excitability, hostility, repetitive movements, self-harm, or lack of restraint
Cognitive: thought disorder, delusion, amnesia, belief that an ordinary event has special and personal meaning, belief that thoughts aren't one's own, disorientation, mental confusion, slowness in activity, or false belief of superiority
Mood: anger, anxiety, apathy, feeling detached from self, general discontent, loss of interest or pleasure in activities, elevated mood, or inappropriate emotional response
Psychological: hallucination, paranoia, hearing voices, depression, fear, persecutory delusion, or religious delusion
Speech: circumstantial speech, incoherent speech, rapid and frenzied speaking, or speech disorder
Also common: fatigue, impaired motor coordination, lack of emotional response, or memory loss
Schizophrenia - Symptoms and causes
which means we may never have an explanation for this horrific murder.
I agree with you, revenge doesn't make sense. I do recall reports that the 12 year old daughter of MS (E) was alleged to have been "creeped out" by DB. Since the alleged attempted abduction in February (before the twins resided in Sherrod household) was just a few doors down from MS I wonder if DB considered ES his "girlfriend" and the February events brought MS attention to the fact that DB's issues were much more serious than she imagined, hence his welcome there was revoked. But I don't see DB's actions with Cash as revenge.
UNLESS, as with the little girl a few doors down, he was going to abduct Cash until someone produced his "girlfriend" and then Cash awoke, screamed caused a ruckus or something that made DB panic and kill him.