My reference for colonial Pennsylvania is _Albion's Seed: Four British Folkways in America_, by David Hackett Fisher. Pennsylvania was a remarkable place, with a lot of very impressive accomplishments. Fisher is full of praise for their Old World-quality infrastructure, much of which is still in use today; he also points out their peaceful coexistence with the American Indians, their roughly equal rights for women, and their rehabilitative prisons, concern for animal welfare, and interest in vegetarianism. But there's also how they refused to bear arms and expected the British military to protect them -- and how they enforced sumptuary laws and the like with corporal punishments, and drove off the Scotch-Irish as being just this side of subhuman.
Admittedly, one could make a very strong case that the Scotch-Irish really were just this side of subhuman. They were lawless, xenophobic, and economically improvident, as well as scornful of the laws and customs of war; they committed savage atrocities in just about every war they engaged in, and were more or less responsible for the Indian Wars, the Mexican War, and the lawlessness of the Wild West. Andrew Jackson, who ethnically cleansed the Cherokees and the other Five Civilized Tribes, is still regarded as a hero in Appalachia today, and his atrocities are either brushed off or openly celebrated.
A large and politically powerful Scotch-Irish population would have poisoned Pennsylvania; the Quakers had the choice between self-destruction and rejection of their principles. They chose to reject their principles -- and then they kept preaching them, completely unmodified, as if they'd never rejected them at all.
I'm Catholic, so it's not surprising that I'd say this, but I'd say that Catholic social doctrine -- with its strict criteria for just wars, and various prescriptions for government and suppression of crime -- is a better choice than denying the necessity of violence and then having to improvise a solution when violence turns out necessary anyways.
I certainly do condemn pacifism, because it doesn't work; but at the same time, I'm pretty thoroughly impressed with most of the Quakers' accomplishments, and I certainly accept the importance of using minimal force and humane policies.
I should also add that I was too hard on the Scotch-Irish in my previous post. _Albion's Seed_ discusses the conditions that shaped them: for 500 years, from the foundation of Scotland to Prince Charlie's rising, every king of England except three invaded Scotland, was invaded by the Scots, or both; and in every one of these invasions, the Scotch-Irish bore the brunt of it.
Sorry to distract from the actual content of your well written post. I simply found this an interesting choice.
This acceptance of personal witness extends to our concept of god. So you often get Quakers discussing what each understands by "god". Some are very biblically influenced and imagine some kind of omnipotent "person", but the concept of a shared spirit that comes from ourselves makes more sense to me. For me "god" is not a person's name, and is not an entity that will be hurt if I don't dignify it with a capital letter.
But I'm very happy for you to do otherwise.
The central tenet of our faith is that there is that of god in every person, so it follows directly that violence against any person is violence against god, and therefore unacceptable. Quakers have put their money and actions where their mouth is - they have always rejected capital punishment, and have been very active in prison reform since the 1800s. They set up the Friend's Ambulance Unit so they could serve during the Second World War. Quakers also recognise that there are many forms of violence, and sponsor the Alternatives to Violence Program (AVP) training for schoolkids.
To get back to the original question about dealing with violent people, of course you need to use force against them, to prevent them doing more violence. But that force should be as non-violent as possible, to make it clear that society's norms are intact. Norway's treatment of Anders Breivik is inspirational - it is clear that his incarceration is in no way violent - there is no element of revenge, much to many people's fury. But imagine a Breivik sympathiser - seeing a civilised society treating him in a civilised way leaves very little to spur them on to similar acts.
And what about Daesh? They have successfully used resentment against the violence done to muslims to build a society where violence is the norm. This is not sustainable - it won't be long before the flood of recruits reverses, as they discover what arbitrary violence means to them and their loved ones.