Source: The Stone That Never Came Down (1973), Chapter 23 (p. 180)
“Even the young are killing each other.”
Upon hearing that neo-fascist youth had attacked other youth reading a communist paper, as quoted in "From the archive, 30 September 1978: Pope John Paul I dies of heart attack" by George Armstrong, in The Guardian (30 September 2013) http://www.theguardian.com/theguardian/2013/sep/30/pope-john-paul-1-death-vatican
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Pope John Paul I 10
263rd Pope of the Catholic Church 1912–1978Related quotes
“In the end, it is ideas for which people kill each other.”
Introduction, p. 11
Think (1999)

“Bodily decrepitude is wisdom; young
We loved each other and were ignorant.”
After Long Silence http://poetry.poetryx.com/poems/1432/
The Winding Stair and Other Poems (1933)
Context: Speech after long silence; it is right,
All other lovers being estranged or dead,
Unfriendly lamplight hid under its shade,
The curtains drawn upon unfriendly night,
That we descant and yet again descant
Upon the supreme theme of Art and Song:
Bodily decrepitude is wisdom; young
We loved each other and were ignorant.

“We will not learn how to live together in peace by killing each other's children.”
“Dad, how do soldiers killing each other solve the world's problems?”
Source: Calvin and Hobbes: Sunday Pages 1985-1995: An Exhibition Catalogue

“If there is a decree from Ranee [Rajya Laxmi], we must kill each other.”
To nephew Jang Bahadur Rana as quoted on Yadav, Pitambar Lal (2000); Nepalko Rajnaitik Itihas; publisher - Bijay Kumar (Saharsa); page: 157 http://nkcs.org.np/cdc/library/opac_css/index.php?lvl=notice_display&id=5631
Variation: In the Rājakāja if circumstances arises, we must kill each other without hesitation. Quoted on J.B.R., Diamond Shumsher (1970); Seto Bagh; publisher: Sajha Prakashan, Lalitpur
Quote
“You're basically killing each other to see who's got the better imaginary friend.”