Revealing mistake: When Jessica visits the basement, there's plenty of light everywhere. She then turns off the light, but there's still plenty of light. However, when Adam Frobisher shows up half a second later, the whole place is now in darkness to allow for all the mystery and the intrigue.
Revealing mistake: When Emma hits a man using the flower pot, Angela Lansbury's stand-in is a much younger stunt double.
Revealing mistake: When Jessica is knocked down by the kids, she is replaced with a younger stunt double.
Revealing mistake: When Jessica is meeting with Congressman Dan Keppner, there are two windows with the Capitol building behind. One can see it's a painted backdrop, most noticeably the window on the left with the garden view.
Revealing mistake: All of the exterior shots of Jessica and Hagarty are wide shots with very obvious stand-ins. Jessica is thinner and younger, while short-haired Hagarty's double is younger, chubbier, and has long hair around the nape.
The Murder of Sherlock Holmes - S1-E1
Revealing mistake: In part 2 of the episode, when Jessica is mugged, Angela Lansbury is replaced with a younger and thinner stunt double twice: when she runs away and when she protects herself against the wall.
Revealing mistake: When Jessica rides on the motorcycle, one can see it's a much younger and thinner stunt double who doesn't resemble Angela Lansbury.
Revealing mistake: After Denton falls off his horse, Jessica arrives and jumps off hers. Her stand-in is a much younger and thinner woman who looks nothing like Angela Lansbury.
Death Takes a Curtain Call - S1-E9
Revealing mistake: When the cops arrive at supper time at Jessica's door, the background is a blatant painted backdrop. (00:31:45)
Answer: In a flashback scene, it was shown that he had a violent temper, so it stands to reason that he finally killed her in a fit of rage.