Short of assassinating FJ himself, which would be an extremely difficult task (and a rather pointless one) just killing a high-ranking Austrian official would not necessarily trigger war, especially not if Franz Ferdinand has decisive input in the decision-making.
The exception would be a botched assassination in Sarajevo where Sophie Chotek is killed and Franz Ferdinand is not; in that case, he would be very likely to want vengeance.
Anyway, Franz Ferdinand was, apparently, unconvinced of the wisdom of going to war with Russia over Serbia, which most understood would be the case if Austria attacked Serbia.