And as we know, there are no such thing as coincidences. I can smell the fear via TCP\IP
Put it in the water supply at an opportune time - many thousands killed in a short period of time. Similar to the plot from Dark Knight with Scarecrow.
Wherever it suits your goals. A huge false flag like that instills fear and when people are afraid, they readily abandon their freedom in exchange for the promise of security. In reality, the people they are giving their freedom to are the same people that attacked them in the first place.
I'd say that depends how much was used, and where in the supply system it was injected. I think it would be easy to inject a 55-gallon drum worth into a local distribution node and kill basically everyone that drinks the water coming from that node.
Carfentanyl is just an enantiomer of Fentanyl, yet it's over 100 times as potent as Fentanyl.