How does a seemingly normal 40-year-old family man lose all sense of perspective and transform into an enraged murderer who callously took the life of an innocent young woman, an accountancy student, making her way home on a Saturday evening?
Were these dormant, deviant personality characteristics or psychopathic traits, hidden from view waiting for the perfect conditions to reveal themselves?
Was this a man whose levels of stress and anger exceeded his coping abilities, resulting in the primal and depraved actions that led to the death of Jastine Valdez?
To the outside observer, Mark Hennessy was a married man with two young children.
He was helpful to his neighbours. He played ball in the garden with his daughter. He was in stable employment.
He recently bought a new car for his wife following the birth of their second child. These are perfectly normal behaviours reflecting normal human life.
While he'd had a couple of scrapes with the law in the past, these were relatively minor and certainly did not marry with the crazed actions of last weekend.
So how did this happen? What caused Hennessy to abdicate reason, humanity, empathy and morality and violently abduct a young woman in front of witnesses on a bright summer's evening?
We know he was on bail having been apprehended on suspicion of drink-driving, damaging vehicles and leaving the scene of an accident.
We have also heard he had recently become a heavy drinker who was ejected from pubs in Bray.
He was known to have taken cocaine in recent times and following his actions on the night of Saturday, May 19, there are reports of his drinking to excess and seeking to access cocaine.
Alcohol is known to be a central nervous system depressant with euphoric effects (such as elation and feelings of power), disinhibition (permitting us to behave in ways that are unthinkable when sober) and numbing of the senses.
Cocaine is a stimulant that increases energy and mental alertness, providing an intense energetic high of brief duration.
This is a potentially lethal combination of narcotics that causes agitation, irritability, paranoia and aggression, resulting in an aggressive, paranoid individual with poor judgment and disinhibition.
While we don't know what emotional turmoil Hennessy was experiencing prior to his frenzied attack and killing of Jastine Valdez, his behaviour at the time was clearly aggressive and without regard for the consequences for Jastine, her family or himself and his family.
Were narcotics a significant factor in determining his actions? They certainly appear to be an aggravating factor.
The facts indicate Hennessy was spending less time with his family given his increased frequenting of pubs and excessive consumption of alcohol, which resulted in distancing himself from those dear to him and in whom he could confide. He became more isolated and perhaps even alienated from his supports. It has also been reported he was a shy man who wouldn't readily reach out to others.
Did he reject his family's attempts to reach out to him and thereby make himself inaccessible? If so, why?
Was he dealing with a mental-health issue that was either poorly controlled or undiagnosed that caused him to self-medicate with this nasty cocktail of drugs?
Was this due to a fragile self-esteem, and an inability to reach out to others? Did recent life events challenge his masculinity and render him feeling powerless in his family? Did he then try to regain that power elsewhere?
We have seen video footage of him recently attempting to engage with a group of women on a night out. We also know he was using the dating app Tinder. Were his attempts to engage rejected, reinforcing a sense of inadequacy and fuelling his anger? Was it a combination of all of the above?
We know that for those in emotional and acute distress, both medication and social supports are essential to ameliorate violent and self-destructive urges.
Those who have the skills seek help from family and friends, and access professional/medical help. The capacity to reach out and access help in a time of crisis has been well-established to be a positive prognostic indicator for mental health.
Hennessy found Jastine Valdez walking on a quiet country road. He had a knife, he violently assaulted her and bundled her into his car in daylight in front of a witness.
Did he want sex to satiate his urges and provide him with an outlet for his agitation and arousal and help calm his anxiety by providing the illusion of intimacy, companionship and support? Or was he enraged and angry with women for not meeting his needs?
Jastine did all in her power to escape, tragically to no avail. Did killing her assuage his agitation and satiate his needs? It seems not, as his car was seen driving at speed along roads familiar to him. His car was seen in a number of familiar locations following Jastine's killing.
He frequented a pub to obtain cocaine and alcohol after killing Jastine. He was clearly overwhelmed and in search of some means of quelling his agitation and arousal.
He did not flee the scene, nor did he hide. While he hid Jastine's body, he did this in haste in an area where it was highly likely she would be discovered.
The grim reality of his despicable acts had dawned on him when he phoned a family member and said he would not see them again. At that juncture the effects of the narcotics were likely diminished and he was confronted by the tragic consequences of his actions, which would prove too much for him to live with.
Indeed, when cornered by gardai, he attempted to take his own life, using his knife, in a final act of violence, unable to live with the tragic consequences of his actions.
His remorse was reflected in a blood-soaked note indicating where Jastine's body was and a single word inadequately, and hollowly, acknowledged the devastation he had visited upon Jastine, her family, his wife, children and family: "Sorry".
Dr Patrick Randall is Director of Forensic Psychological Services Ltd