I assume this thread is not serious.

Poisoning is easy or an infection. Faking drug overdose is relatively easy. I wonder why homicides don't use these methods commonly.

Injecting potent poison or a fatal illness by leaving tipped sharp objects soaked in it like sharp pieces of wood hidden in a seat so that nothing raises suspicion.

Airguns with bullets made of ice with a tiny carrier of poison/microbes in the tip like a splinter soaked in it so that it gets embedded on hit and transferred. Ice is only to propel that splinter, and ice melts and disappears. I read about assasinations this way. Ice only hard enough to break on impact and not make a hole in the body so that nobody cares.

Best are ones that start producing symptoms slowly so that nothing is suspicious. Rabies is relatively easy to extract and almost certainly fatal. They will ask for dog bites but they would say no and everyone would waste time until it is too late. Very long term suffering and low risk is asking local junkies for blood samples for drug money and using HIV.