We are, after all, only human, my friend.
Only one, by your lights, has been perfect.
And he forgives. Godspeed.
Much in the same way that some of the Colonial Pastors urged their congregations to take up arms and "kill" for their liberty.
Augustine, Aquinas, Calvin, and Luther (among others) have tackled this issue much better than I could. Christ himself urged his disciples to get swords, even if they needed to sell their cloaks to buy them the night before he was crucified. (Luke 22:36) In a fallen and sinful world, force is sometimes the only solution; indeed, the one MORAL response to pure, murderous, and irrational EVIL.
The case in this passage is the proper response to an insult, or very minor injury (a slap on the face). Not to a threat on one's life, family, or country.
or following the interpretation "thou shalt not kill"
This is correctly translated, "Thou shalt not murder" as has been stated above.
then in the end this may be precisely why your God allows one to sin, but then ask forgiveness.
Shall we then sin, that grace may abound? God forbid.
We are, after all, only human, my friend. Only one, by your lights, has been perfect. And he forgives. Godspeed.
Nevertheless I live, yet not I, but Christ liveth in me, and the life which I now live in the flesh I live by faith in the Son of God who loved me and gave himself up for me.
Forgiviness is no license to sin.