Nicotine is highly toxic to cats.
Sadly, lacking an opposable thumb, it is very hard to get them to start smoking; both matches and lighters are beyond their abilities.
Nil desperandum. Technology offers an answer, in the form of the new BorgCo Cat Vaper. Using components from a standard "electronic
cigarette", plus other off-the-shelf devices, all you need to do is to fill it with the cat-food flavoured liquid and charge the battery, then leave it in a place that cats frequent.
When kitty approaches the box, it proceeds to emit a cloud of fine mist with an attractive odour, encouraging said feline to inhale deeply, and sucking a hopefully lethal dose of nicotine into its respiratory tract.
All that's then necessary is to dispose of the mortal remains ...