Morally is defined, by Merriam-Webster, as
relating to principles of right and wrong in behavior; expressing or teaching a conception of right behavior; or a conforming to a standard of right behavior.
This being said, one could justify that the collective murder which takes place in Shirley Jackson's short story, "The Lottery," could be morally justified.
In regards to the society, and surrounding societies, the lottery is the "right thing to do" according to the townspeople. They live by the rule "Lottery in June, corn be heavy soon." The townspeople believe that the lottery is conducted according to a "standard of right behavior." Those who have been part of the community since its establishment, and born into the community, have conformed to the annual ceremony. They simply take part because it is part of their history and part of their culture.
While not many speak out against the lottery, ending the lottery has certainly been discussed. The only time the lottery really comes under fire is by the one who "wins" the lottery. That being said, Tessie does not actually speak out against the lottery itself; instead, she simply states that the drawing was not fair. No one seems to be troubled by the fact that the winner ends up being stoned to death.
Therefore, one could easily justify that all see the lottery as a part of "right behavior." The lottery has been around longer than many of the citizens have been alive. It is simply a part of their culture.
Outside of that, one could argue that the communal murder is not morally justified. Today, many would have a large issue with a person being stoned to death for crops. Unfortunately, not all countries would agree with it being wrong. There are still many countries which stone people today.
As of September 2010, stoning is a punishment that is included in the laws in seven countries including Saudi Arabia, Pakistan, Sudan, Iran, Yemen, the United Arab Emirates, and some states in Nigeria.