1/6  You have certainly raised an interesting issue and I’ve put quite a bit of time into it before answering you. This is really not a Catholic issue; the majority of Christians hew to the same approach Catholics do, even though many of the liberal sects have departed from natural law theory to justify other things they are doing. Small sects hew to your approach, and conscientious objection is a minor but serious component of American beliefs.

First, the Scriptural references, which I provide with a short commentary from a mainline Protestant commentary - just so we understand this is not a uniquely Catholic view of the texts:

Matthew 5: 43 Ye have heard that it hath been said, Thou shalt love thy neighbour, and hate thine enemy. 44 But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;