The laws ... which forbid to wear arms, disarming
those only who are not disposed to commit the crime which the laws mean
to prevent. ... It certainly makes the situation of the
assaulted worse, and of the assailants better, and rather encourages
than prevents murder, as it requires less courage to attack unarmed than
armed persons.
Cesare Beccaria (1764, Essay on Crimes and Punishments)
A similar quote is often attributed to Thomas Jefferson, but he merely included the above passage in his "Legal Commonplace" Book. That quote that is often attributed to him is below. Thanks to "Josh" in the comments section for bring this to my attention.
Laws that forbid the carrying of arms ... disarm only those who are
neither inclined nor determined to commit crimes. ... Such laws make
things worse for the assaulted and better for the assailants; they serve
rather to encourage than to prevent homicides, for an unarmed man may
be attacked with greater confidence than an armed man.