Searching for the Cause: Psychological Disorder?

If a killer has a psychological disorder, could that be why they kill?

Research shows that brain development could play a role in determining the possibility of a serial killer. At a certain time, the brain develops in a particular way that controls the statistical likelihood of behaviors occurring under certain circumstances, such as serial killing. If this development is interrupted or stunted, it could likely be the reason why some people do end up killing.
The most broadly recognized mental disorder associated with serial killing is Antisocial Personality Disorder (APD), which is closely related with psychopathy.
**Psychopathy is not a clinical diagnosis, but rather a developmental disorder. Not all psychopaths are serial killers.
Borderline Personality Disorder (BPD) is another disorder associated with serial killing that includes emotional instability, anxiety, and psychotic-like symptoms that can cause people to be suddenly paranoid or suspicious of others. It can result in zero empathy and is often accompanied with impulsive aggression.
Narcissistic Personality Disorder (NPD) can result in a person feeling no empathy, and having brain abnormalities, and is common in serial killers.
Schizophrenia can cause those infected to become violent, especially when hallucinating. For example, the notorious Richard Chase (Vampire Killer of Sacramento) was a diagnosed as a paranoid schizophrenic. Those suffering from schizophrenia usually have jumbled thoughts (unlike premeditated murders); enlarged lateral ventricles, depleted myelin sheaths, and abnormal clusters of neurons.
Other disorders have been implicated in the possibility of being related to serial killing or violent actions in humans. If we could discover the way these disorders work, could we be able to stop them from causing these feelings of violence in those that they inhabit?

If a serial killer has a serious mental disorder that impairs their judgement, can they truly be held accountable? Some of the killers know good and well what they're doing, but is there anything that we could have done to prevent them? Or at least help them? Should we feel sympathetic for them, or focus on their crime? The sentence usually depends on the degree of the disorder and how much it visibly affects the person, but should we take disorders into account when judging?