In A Christmas Carol, what is the meaning of the quote, "Will you decide what men shall live, what men shall die?"

This quotation comes in the middle of a rebuke Scrooge receives from the Ghost of Christmas Present. Scrooge, moved with pity toward Tiny Tim, asks the Spirit if the child will live. The Ghost replies that within the year, Tiny Tim will die if the "shadows remain unaltered by the Future." After pronouncing that fact, the Spirit casts Scrooge's words of the previous day in his face: that if Tiny Tim is going to die,...

This quotation comes in the middle of a rebuke Scrooge receives from the Ghost of Christmas Present. Scrooge, moved with pity toward Tiny Tim, asks the Spirit if the child will live. The Ghost replies that within the year, Tiny Tim will die if the "shadows remain unaltered by the Future." After pronouncing that fact, the Spirit casts Scrooge's words of the previous day in his face: that if Tiny Tim is going to die, he should do so "and decrease the surplus population." Those are the words Scrooge used when asked to donate to the poor and destitute. Scrooge pointed out that the poor could go to debtor's prison and the workhouse. The charity men replied that some would rather die than go there, to which Scrooge replied, "If they would rather die . . . they had better do it, and decrease the surplus population."

The Spirit, after repeating Scrooge's words, challenges Scrooge to find out "What the surplus is, and Where it is." By this he means that Scrooge is in no position to determine what part of humanity constitutes the "surplus population." The Spirit compares Scrooge to a bug dining on a leaf and despising the other bugs who survive, though hungry, in the dust. To an omniscient being like God, all humans could appear as mere bugs. Humans, whether they are wealthy or poor, are as consequential or inconsequential as their brothers in such cosmic terms. The Spirit uses this speech to burst Scrooge's pride in his station in life and to help him realize that his wealth doesn't make him worthier than his brothers, and it certainly doesn't give him the right to determine who should live and who should die. 

The Ghost also says that Heaven may decide that Scrooge is "more worthless and less fit to live" than the surplus population Scrooge scorned. This foreshadows the next stave, where Scrooge views his own death and comes to terms with his mortality.

The Spirit's rebuke is effective. At his words, Scrooge is "overcome with penitence and grief." At this point Scrooge begins to change.