There are a couple of factors here.

One was that during and just after WW, British juries had been reluctant to dish out guilty verdicts on serving soldiers and veterans, even when caught in flagrante. This to the annoyance of masny judges, lawyers, police oficers and politicians who saw it as undermining the justice system. The post-War Labour government was also lenient in handing out death sentences.

Then there was the matter of the insanity plea, which was becoming more and more popular among defence lawyers as a way of avoiding execution for their clients. It must be borne in mind that in the following year the serial murderer John Christie quite cynically attempted to use an insanity plea. At this time, juries were very ready to accept this plea, though in Chrisites' case they were not swayed by it. But at this point, it was common for both juries asnd judges to recommend clemency if the accuseds' state of mindc was in question.

It's more than possible that Maxwell Fyfe was sending a message to judges and juries to stop letting themselves be gulled by every Harry Hogwash and Sally Sob-Story who came before them. In conservative circles, the death penalty was considered fundamental to the effectiveness of the system- -not as a deterrent, but as a proper and God-ordained punishment for the most heinous of crimes "a life for a life".