Just because you can, doesn't mean you should. Knowledge of potentially destroying large swaths of humanity cannot be categorized as good.
QFT.
Why not? Understanding how these things work often holds the key to harnessing their power later, or dealing with the consequences.
There are so many examples of research like this, nuclear fission, antibiotics, explosives etc that have both good and bad effects. The threat of a devastating natural pathogen is very real, maybe this knowledge and subsequent investigation will help find a prevention or cure.
The trick is, we still have a LOT to learn about virus's, and we can still learn things from more mundane virus's, or by creating less deadly virus's to examine. There's no reason to create an airborne death virus when we can examine avian flu and some other airborne (non-lethal) virus.
That's not to say we should never create/examine a virus like this, but I'd feel safer if we did it on a different planet, or only after every-other virus was completely known.
I don't think this should be published for everyone to see, only those who want and are trusted enough to conduct further research, but I have no problems with this being researched in the first place.
That's not exactly how science works though. And IF a virus exists, and there's a .0000001% chance it will escape, that's scary enough.
There was a woman, Henrietta Lacks, who had cancer. She died, but her cancerous cells were saved, and grown in a lab. They were used to test the polio vaccine, among others. The problem is that they grew so well, and were so abundant, that 50 years later a majority of cell-lines in America had been contaminated and then overrun with the HeLa (Henrietta Lacks) cells. Most of the cell lines that they thought beloned to individuals, actually had been overrun by cells from a single person.
Now, imagine that scenario, only instead of a cancerous cell line used for testing drugs, it was a virus that killed. Scary, isn't it? And the HeLa cells weren't even airborne!