Once the cat's out of the bag, it will be an arms race between biological attack and biological defense. Rich people and politically connected people will be vaccinated against the murder-viruses. And those are the only ones worth spending more than a brick of ammo to kill.

It's far easier to take an already-deadly disease that kills indiscriminately, modify it to have shorter incubation time and higher kill rate (so it burns itself out before going pandemic) and shoot it at a target on a spitball or with a Bulgarian umbrella, while they're on their way to meet with everyone else you want to kill.

Either way, that's still leaving biological evidence all over the crime scene, and has the potential to kill a lot of people you weren't specifically targeting. So the preferred methods of assassination will likely continue to be poisons, firearms, and explosives for a long time to come.