I’m still looking over Colin Wilson's History of Murder and I went back to the Ripper section where I found this passage relating to Joseph Sickert’s supposed involvement in the Royal Conspiracy:

Does this mean that Sickert was Jack the Ripper?  Almost certainly not.  Artists and writers may become morbidly obsessed by certain murders, but no artist has ever been known to commit premeditated murder.

Uh, what?  See Pen, Pencil and Poison by Oscar Wilde concerning the poet Thomas Wainewright.  Or Caravaggio in 1606, if we can count a duel as premeditated.