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.