Diminished responsibility is a defence for reducing a sentence of murder to voluntary manslaughter. For example;

Person A suffers from a recognised medical condition, such as schizophrenia, who regularly takes medication. 

Person A then decides to stop taking their medication, leading to their condition gradually getting worse.

Person B is usually nasty towards person A, so in this moment, person A stabs and kills person B.

Assuming the actus reus and mens rea for murder are fulfilled, which would be more than likely… person A would be charged with murder of person B. This would result to a mandatory life sentace.

But, because of the medical condition, person A has a possibility of a defence of diminished responsibility. Without going into the legal path this endows (will explain another time) person A could then have their sentence reduced to voluntary manslaughter. This could potentially lead to a much lighter sentence.
So that’s responsible right? Perhaps.

Schizophrenia is only an example. However, their is plenty of common law that shows conditions such as depression have been accepted as a defence of diminished responsibility. It has been known for someone to have their sentace reduced to as low as 6 years (rather than a life sentace).
So now let us think of a different scenario, person A plans to kill person B. In doing so, they decide that if they get caught, they will soften their landing and safeguard his outcome. 

Over one year, person A claims depression, receives medication, and takes some time off of work. They do not have to strictly have depression, but all this person needs is the imitation of depression, and enough to convince a jury.

Person A kills person B, and shortly after person A is found guilty of murder. But they now have a possibility of a defence! Common law provides precedent for depression to be a valid defence! Previous medical records over the last year can provide evidence, and time taken off of work can suggest the severity of the depression.

This does not mean they WILL have their sentence reduced, but there is a possibility, a possibility that was not available without this approach. 

I want to emphasise that there is a lot more involved with this defence, however in black and white, this is very possible.