Thus, with sound reason, they give great weight to the plea of self-defence which the accused urges upon them.

He is generally the only witness.

His story has to be disproved by circumstantial evidence, if indeed there be any.
Frequently it stands alone as the only account of the homicide.

Thus murder cases are almost always weaker than others, since the chief witness has been removed by death; while at the same time the nature of the punishment leads the jury unconsciously to require a higher degree of proof than in cases where the consequences are less abhorrent.

All this is quite natural and inevitable.