To me, the last line reflects a really terrible view of human nature on the part of Robert Frost (or at least on the part of whoever is narrating the poem).
The last line calls into question the relationships between the various members of the family. It implies that they didn't really care a whole lot about their son. Once they figure out that he is dead they go about their business because they are not the ones who died.
So, overall, this line turns the whole feel of the poem around. From the time that the boy has his accident until the last line, you assume that everyone feels awful. But then it appears they don't care.
This makes the whole poem seem cynical and like it's saying that human beings don't really care about each other.
The last line represents some of the most disturbing elements featured in the poem. After enduring the pain of the boy pleading with the sister to prevent the doctor from amputating his arm, enduring the blood and pain of the ordeal, the final testimony to this episode is that the family ends up moving on with the boy's horrific incident almost an afterthought. I think that the hollowness featured in some emotional relationships is brought out powerfully in the closing line. The idea that the mourning period of a death and its experience are taken in different ways, with different approaches is evoked. The final piece I am left with is the idea that the sensationalist drama of the boy has given way after he dies. There might be a statement about how individuals are gripped by situations, only to move on from them after they have passed with little, if any, reflection on them.
The key to understanding the theme of Frost's "Out, out-" lies in the intertextual reference to Shakespeare's "Macbeth" Act V Sc.5, where Macbeth soliloquizes bitterly on the futility of life after he learns of the death of his wife:
Out, out, brief candle!
Life's but a walking shadow, a poor player,
That struts and frets his hour upon the stage,
And then is heard no more. It is a tale
Told by an idiot, full of sound and fury,
Frost's poem ironically comments on the death of a small boy who dies tragically at such a young age because of an accident when he was sawing wood. His life is compared to a "brief candle."
The last two lines contain the message or moral which Frost wants to convey to his readers. Frost's message is that anything can happen at any time. There is no absolute safety or security for human life. The next minute is not ours and we may be alive one minute and dead the very next minute. The only thing that we can do is to go on with our lives. Just because the small boy died it does not mean that all the others will die in a similar fashion. The death of the small boy cannot be an excuse for inaction. So, the others continue with their work and lives even after the death of the boy:
No one believed. They listened at his heart. Little--less--nothing!--and that ended it. No more to build on there. And they, since they Were not the one dead, turned to their affairs.
"No more to build on there. And they, since they
Were not the one dead, turned to their affairs."
The last line of the poem seems to indicate that the parents have realized that they can do nothing more for their son, so they leave to prepare arrangements for his burial. Some would say that is a more positive interpretation.
Another impression of the end of the poem is that the parents realize that their son is dead and of no more use. Perhaps they were a farm family in times when the boy was another mouth to feed or a laborer. If he died then he can no longer help the family. However, the farm work goes on, and they have to return to their daily lives to continue working as they always have worked.
The end line does take the mood and shift it around. The reader is feeling sorrow for the death of the boy. When the last line is read, the reader is somewhat bewildered by the lack of concern on his parents behalf.