“Death isn't a tragedy to God, only to those left behind.”

Aceldama : A Place To Bury Strangers In (1898) Preface.

Source: Shadow of the Hegemon

Source: Dreams of Steel (1990), Chapter 11 (p. 264)

Source: Dark Age (2019), Ch. 34: Shadows of War; Octavia

Source: Anna Karenina

Variants: One death is a tragedy. A million deaths is just a statistic.
A single death is a tragedy; a million deaths is a statistic.
When one dies, it is a tragedy. When a million die, it is a statistic.
In Портрет тирана (1981) (Portrait of a Tyrant), Soviet historian Anton Antonov-Ovseyenko attributes the following version to Stalin: "When one man dies it's a tragedy. When thousands die it's statistics." This is the alleged response of Stalin during the 1943 Tehran conference when Churchill objected to an early opening of a second front in France.<!-- The book appears to have a footnote sourceing the claim, but I couldn't access it. Could someone please try to scare up a paper copy and have a look at footnote 188? -->
In her review "Mustering Most Memorable Quips" of Konstantin Dushenko's 1997 Dictionary of Modern Quotations (Словарь современных цитат: 4300 ходячих цитат и выражений ХХ века, их источники, авторы, датировка), Julia Solovyova states: "Russian historians have no record of the lines, 'Death of one man is a tragedy. Death of a million is a statistic,' commonly attributed by English-language dictionaries to Josef Stalin."
This quotation may originate from "Französischer Witz" (1925) by Kurt Tucholsky: "Darauf sagt ein Diplomat vom Quai d'Orsay: «Der Krieg? Ich kann das nicht so schrecklich finden! Der Tod eines Menschen: das ist eine Katastrophe. Hunderttausend Tote: das ist eine Statistik!»" ("To which a Quai d'Orsay diplomat replies: «The war? I can't find it so terrible! The death of one man: that is a catastrophe. One hundred thousand deaths: that is a statistic!»")
Another possible source or intermediary may be the concluding words of chapter 8 of the 1956 novel The Black Obelisk by Erich Maria Remarque: "Aber das ist wohl so, weil ein einzelner immer der Tod ist — und zwei Millionen immer nur eine Statistik." ("But probably the reason is that one dead man is death—and two million are only a statistic." 1958 Crest Book reprint)
Mary Soames (daughter of Churchill) claims to have overheard Stalin deliver a variant of the quote in immediate postwar Berlin (Remembrance Sunday Andrew Marr interview BBC 2011) https://www.youtube.com/watch?v=9hP2tpw9XEw
See also Jean Rostand, Thoughts of a Biologist, 1939: "Kill one man, and you are a murderer. Kill millions of men, and you are a conqueror. Kill them all, and you are a god."
In an interview given for the 1983 three-part documentary Der Prozeß by Norddeutscher Rundfunk on the Third Majdanek trial, Simon Wiesenthal attributes the quote to the unpublished auto-biography of Adolf Eichmann. According to Wiesenthal, Eichmann had been asked by another member of the Reich Main Security Office during WWII what they should answer would they be questioned after the war about the millions of dead Jews they were responsible for, to which Eichmann according to his own testimony had replied with the quote.
Misattributed
Variant: The death of one man is a tragedy, the death of millions is a statistic.

Being from Manson's Fight Song of Holy Wood, this is actually a quote from German writer Erich Maria Remarque, also often misattributed to Josef Stalin.
Misattributed

Source: The Heart is a Lonely Hunter