Death by Flamethrower
At the recommendation of my kids upon a request for amusements, I was watching Game of Thrones. I liked the one where Joffrey drank wine that had been infused with the poison they called “the Strangler.” Great scene, well deserved by the character. And this one... especially if he doesn’t know which drink or dish will contain it.
Holmes is evil. His torment should start early.
I would prefer he be hung, drawn, and quartered. Another option could be to have him shot like fish in a barrel, allowing him the same courtesy he gave to his victims.