Yes I really shouldn't have quoted the Leviticus part as that pertained to the society of Israel long ago. But the Noahic covenant still stands. I don't see why God would recant on His word. If you look through various translations from different versions, verse 6 specifies the life of a person.
NIV: “Whoever sheds human blood, by humans shall their blood be shed; for in the image of God has God made mankind."
ESV: “Whoever sheds the blood of man, by man shall his blood be shed, for God made man in his own image."
KJV: "Whoso sheddeth man's blood, by man shall his blood be shed: for in the image of God made he man."
NASB: “Whoever sheds human blood, By man his blood shall be shed, For in the image of God He made mankind."
and so on.
I can understand 'accounting' as in the person who has killed someone must give an explanation of what happened, because not all deaths are deliberately caused.
'Love your enemies' isn't about civil justice though? It's a command given at the personal level, that's my understanding of it. Also it's not in contradiction of God's original edict to take the life of a murderer. Someone who is my enemy isn't necessarily a murderer. There have been cases where parents have forgiven the murderer of their child. That's separate from whether the murderer should or shouldn't receive the death penalty. Our government may bring in laws that murder requires the death penalty (or some other form of punishment), this is a separate matter.
I mean, that's my understanding of it. I was very anti death penalty till recently, now I think it has a place.