This is a slippery slope because of the society presented in the story. On the one hand it certainly can be viewed as a collective act of murder, but perhaps if it were viewed as an act of a society's culture it might not.
The world is made up of a plethora of different cultures who prescribe to different sets of moral codes. What is taboo in one society might not be viewed as such in another. For example, in American culture we keep dogs as pets and are deeply offended when we hear stories of people treating animals cruelly. There are some Asian cultures where dogs are a delicacy and no one bats an eye when a dog is killed and served as a dish at meal time.
The same parallel can be drawn in the Lottery. Our society would view this as a collective act of murder, but supposing there was a society like this where these types of rituals were commonplace and viewed as a cultural norm it might not actually be considered a murder.
A sound argument can made in either case.
Yes, it can certainly be viewed that way for everyone is complicit in the act. Indeed, the main preoccupation of the townspeople is not moral outrage or even a meditation on the rightness of doing this act. It has been done in the past; the children have been raised to continue the tradition. No one considers this might be wrong, but everyone hopes he or she is not the "one" who will be stoned. Does anyone not throw a stone? Does anyone say "let's stop for a while and rethink this act we are about to commit?" No, and for that reason all participate, making the stoning very much an act of collective murder.