The statement is based on an instruction given by Arnaud Almaric back in 1209.
Drawing upon a Wiki article ...
Arnaud Amalric to one of his minions, at the sack of Beziers in 1209. Amalric was a papal legate sent by Pope Innocent III to try to convert the heretic Albigensians. He failed, and then slaughtered them. At Beziers, one of his men asked him, "How do we distinguish the heretics from the true Catholics?" Amalric said, "Kill them all. For the Lord knoweth them that are his."
That's where we get the "Kill 'em all" comment.
Our Donald is fond of going on about killing the families of terrorists. His followers, especially the Eighty Percent, sucked that down like Obama's minions slurped the purple SEIU Kool-Aid.
It isn't a particularly 'Christian' view, but that doesn't bother the Eighty Percent.
The Trumpkins really like this Internet meme; they seem to think that Mattis would be the kind of SecDef that would do this kind of thing 'because he's tough.'
The Trumpkins seem to think that James Mattis is modern day incarnation of Arnaud Almaric. He is not. He has this to say about killing non-combatants, including families of terrorists:
"While Trump has espoused killing the families of terrorists, if necessary, Mattis wrote in a 56-page questionnaire: “The killing of non-combatants in a war against a non-state enemy violates Common Article 3 the Geneva Conventions. Legal questions aside, it is my view that such actions would be self-defeating and a betrayal of our ideals.”
If the Trumpkins are expecting a SecDef who will advocate for the slaughter of Muslims around the world, they are going to be disappointed.
We are not going to get moral leadership from Trump and his circle of sycophants. We stand to get that from James Mattis. Mattis, for all the hubris surrounding him - most of it generated by an ignorant press and an even more ignorant public - is well-read, thoughtful, and fully aware of the cost of prosecuting a war. Trump - Ol' Heel Spurs - thinks it's kind of like banging your way through college and not catching any nasty social diseases.
All that aside, slicing off that fellow's testicles - if it's even true - has not in the least dissuaded radical Islamists from attacking Russia or Russians:
Radical Islamists from Chechnya and other North Caucasian republics have been held responsible for a number of terrorist attacks throughout Russia, most notably the Russian apartment bombings in 1999, the Moscow theater hostage crisis in 2002, the Beslan school hostage crisis in 2004, the 2010 Moscow Metro bombings and the Domodedovo International Airport bombing in 2011.
And how well did that Afghanistan experience work out for the Soviets and the Russians?