Sure, it could be a weapon. Not all weapons work as hoped, SARS *could be* one of those. Despite it's low deathrate for a weapon, it spreads fast and makes it's victims very sick. It is now thought that it causes permanent scarring in the lungs of it's survivors and many recovering victims suddenly got really sick again.
I don't think it is a weapon, but if it turned out to be a weapon(or meant as one) it wouldn't be too surprising. It does have most of the qualities that one would desire in a Biological weapon.