Should understanding everything must lead to forgiveness?

According to Robert Sapolsky in Behave: The Biology of Humans at Our Best and Worst:

It is a mistake to think that understanding everything must lead to forgiveness.

The fact that we can explain a horrific act of murder through neurobiology should not necessarily mitigate the sentence. A young man commits a terrible act under the influence of impulses, and neuroimaging reveals a deficiency in his prefrontal cortex (PFC) neurons. This leads to a dualistic temptation to consider his behavior more ‘biological’ or ‘organic’ in some vague sense than if he had committed the same act with a normal PFC.

However, the horrific impulsive act of this young man is unequivocally ‘biological’—with or without a PFC.