And if the civil authorities give that task to you? Say, as a member of the military, what then? Your statement suggests that Christians cannot be soldiers if you extend that logic out.
Then you are acting as an agent of the State and are authorized to punish and kill in the name of the State, and I do not believe that this is a sin in God's eyes, unless you abuse your authority.
Here is a little exchange between Jesus and some soldiers in Luke 3:14
And he said unto them, "Do violence to no man, neither accuse any falsely; and be content with your wages."
If Jesus told them to be content with their wages, then He obviously did not intend for them to leave the military. So how does this reckon with His saying, "Do violence to no man."
By my reckoning, the word "violence" in the above verse indicates to "terrify" or "intimidate". Note also that Jesus bade them not to falsely accuse anyone. I think that is in the same spirit as the use of the word "violence" in the verse.
Certainly as soldiers, they had to kill in the heat of battle, but they were not to abuse their power or authority.