“It is the enemy you underestimate who kills you.”

Attributed by Gavin McInnes (AKA Miles McInnes) of Rebel Media in the satirical video "Why Miles McInnes is moving back to Canada: Trudeau's 10 Greatest Quotes!" at 5m45s number 9/10 https://www.youtube.com/watch?v=_tpW1m-8yVQ&t=5m45s", along with accompanying article as Gavin McInnes “If you kill your enemies, they win”: Justin Trudeau’s greatest quotes! https://www.therebel.media/justin_trudeau_s_greatest_quotes released the same day, then repeated by Daniel Greenfield in his article "Surrendering to ISIS is the Only Way to Defeat It", Canada Free Press (31 March 2016) https://canadafreepress.com/article/surrendering-to-isis-is-the-only-way-to-defeat-it. A Reddit thread "Any evidence that Trudeau said 'if you kill your enemies, they win'?" https://www.reddit.com/r/canada/comments/68iwe3/any_evidence_that_trudeau_said_if_you_kill_your/ questioning the veracity of the attribution was created in April 2017. No corroborating evidence was found to support the "McCinnes brothers" claims that Trudeau said this, yet many others have repeated it as fact. It is possibly a paraphrasing of Trudeau's February 2016 quote about elevating enemies.
Misattributed

Source: Kill the Dead

Source: Lullaby (2002), Chapter 22

Ender's Shadow

Source: The Godfather

Lookstein, Haskel: "Were We Our Brother's Keepers?" New York: 1985.

Watership Down (1972)