1. Life, human life, is the most precious thing in all creation. It is the crowning jewel of God's creation and worth any sacrifice (including the sacrificial death of his Son, Jesus) to preserve, secure and safeguard.
2. Men are created in the image of God, to be like God. It is only in Christ that we realize this potential. God is cast as a peaceful, loving, gentle, kind Father most of the time. He is also a mighty warrior, wielding a sword and the ultimate conqueror. Fire and brimstone of judgment and wrath are every bit as much 'godly' as grace and peace.
3. Jesus, who came as a Lamb to be slaughtered for our sins, will return as a warrior to destroy those opposed to him.
It's my belief that our Amish friends have idealized one particular facet of Christ and elevated it to a position above the other facets of Christ. That to mimic the humility and gentility of Jesus...remianing silent as they are beaten and killed, is somehow superior to defending oneself (as David did). That's not uncommon, especially in people who believe it is somehow more righteous to ride in a horse drawn buggy and live in a house with no electricity than to enjoy the God given conveniences of modern America.
Whatever, the Amish are not a 'standard of righteousness' that we all have to look toward to gauge our holiness.
I am fairly biblically literate and don't see any biblical prohibition against actively preserving the life that God gave me and the lives that He has given to my care. In fact, I see that as one of my responsibilities.
Armed, righteous men have been an instrument that God has used to do some of His greatest work on planet Earth.
Check out Revelation 19:11 and one of my favorites, Psalm 18, which reads (in part):
For who is God, except Yahweh? Who is a rock, besides our God,
32 the God who arms me with strength, and makes my way perfect?
33 He makes my feet like deer's feet, and sets me on my high places.
34 He teaches my hands to war, so that my arms bend a bow of bronze.
35 You have also given me the shield of your salvation. Your right hand sustains me. Your gentleness has made me great.
36 You have enlarged my steps under me, My feet have not slipped.
37 I will pursue my enemies, and overtake them. Neither will I turn again until they are consumed.
38 I will strike them through, so that they will not be able to rise. They shall fall under my feet.
39 For you have girded me with strength to the battle. You have subdued under me those who rose up against me.