Source: The Dark World (1954), Ch. 9 : Realm of the Superconscious
“A sword by itself does not slay; it is merely the weapon used by the slayer.”
Seneca is here describing arguments used by 'certain men,' not stating his own opinion.
Alternate translation: A sword never kills anybody; it is a tool in the killer's hand. (translator unknown).
Source: Epistulae Morales ad Lucilium (Moral Letters to Lucilius), Letter LXXXVII: Some arguments in favor of the simple life, Line 30
Original
quemadmodum gladius neminem occidit: occidentis telum est.
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Seneca the Younger 225
Roman Stoic philosopher, statesman, and dramatist -4–65 BCRelated quotes

“The mere existence of atomic weapons implies the possibility of their use.”
Quoted in "The arms race: a programme for world disarmament" - Page 297 - by Philip John Noel-Baker - Political Science - 1960

“What necessity for a sword to slay the lover, when a glance can deprive him of half his life!”
In A Century of Ghazels, or. a Hundred Odes, Selected and Translated from the Diwan of Hafiz (1875), p. 77; quoted with a slight change in Love: A Book of Quotations (2012), ed. Ann Braybrooks, p. 71
“Knowledge is a better weapon than a sword.”
Source: Raven's Shadow

Go Rin No Sho (1645), The Ground Book

A New System of Sword Exercise for Infantry (1876)
Context: The recruit must be carefully and sedulously taught when meeting the enemy, even at a trot or canter, to use no force whatever, otherwise his sword will bury itself to the hilt, and the swordsman will either be dragged from his horse, or will be compelled to drop his weapon — if he can. Upon this point I may quote my own System of Bayonet Exercise (p. 27): —
"The instructor must spare no pains in preventing the soldier from using force, especially with the left or guiding arm, as too much exertion generally causes the thrust to miss. A trifling body-stab with the bayonet (I may add with the sword) is sufficient to disable a man; and many a promising young soldier has lost his life by burying his weapon so deep in the enemy's breast that it could not be withdrawn quickly enough to be used against a second assailant. To prevent this happening, the point must be delivered smartly, with but little exertion of force, more like a dart than a thrust, and instantly afterwards the bayonet must be smartly withdrawn." In fact the thrust should consist of two movements executed as nearly simultaneously as possible; and it requires long habit, as the natural man, especially the Englishman, is apt to push home, and to dwell upon his slouching push.