"The Lottery" by Shirley Jackson is one of the most-read short stories in literature, at least in America, and there is a good reason for that. This story illuminates the tendency of human nature to adhere mindlessly to whatever traditions or beliefs we have practiced and held.
In this story, the tradition is a lottery, and of course one of the great ironies of the story is that a lottery is generally thought of as a positive event, and the winner is considered to be quite lucky. Obviously this is not the case here, though you have to read carefully to sense the tension on this lovely, sunshine-y day.
The so-called winner of this lottery is stoned to death as soon as he (or in this case she) is chosen. Everyone who is not chosen, in other words everyone who escapes this fate, immediately throws the stones they have gathered until the "winner" dies. The only word for this act is murder, and your question is a valid one.
On one hand, if everyone in town, even the victim's loved ones, participate in something the victim knew might happen, perhaps it should not be called murder. On the other hand, what else can it be called?
The key to your question is in the words "morally justified." This phrase implies that some kind of greater good might be gained by this death; if so, perhaps it should not be called a murder. This is the kind of thinking which is applicable to killing in battle or in an act of self-defense. In general, those deaths are not called murder.
In this case, there does not seem to be any need for anyone to die. We can surmise that perhaps once, many, many years ago, this ritual was part of a ceremonial sacrifice in order to ensure a good crop or some other cause which might possibly be justified. Today, however, there is no apparent connection between the death of a citizen and some benefit for the greater good of the community.
Instead what we find is that this ritualistic murder, along with all the other trappings of the lottery, is nothing but meaningless, mindless tradition.
The original paraphernalia for the lottery had been lost long ago, and the black box now resting on the stool had been put into use even before Old Man Warner, the oldest man in town, was born. Mr. Summers spoke frequently to the villagers about making a new box, but no one liked to upset even as much tradition as was represented by the black box.
Tessie Hutchinson dies a meaningless death which is unconnected to any kind of necessity and therefore it has no moral justification. It is murder, and it is inexcusable. The only possible way to get a moral justification for this act is the certainty that if someone else had picked the paper with the black mark, even her young son, Tessie would have been just as quick to throw stones at them as the others were to throw stones at her.
The simple word "murder" says it all; justifiable deaths are not generally given this label.