No Brother or Sister in Christ can “curse” us or be an “enemy,” nor can they “despitefully use” us or “persecute” us.
The reference in 2 John is reasonable, but your Romans reference is specifically talking of ALL people, not just Christians, and even says to feed and give drink to these enemies of God. “Recompense to NO MAN evil for evil...Provide things honest in the sight of ALL MEN...live peaceably with ALL MEN...THEREFORE (what follows is still for ALL MEN).
Sometimes it is helpful to look at more than one translation of a passage to get a better idea of its meaning. I pretty much interpret for myself what I read in the Bible, though I sometimes seek guidance from others when I don’t quite understand something. I’m not saying I’m always right, and often I interpret what I read differently than many (or even the majority of) others, but I do my best.
Rom 12:17-21
17 Never pay back evil for evil to anyone. Respect what is right in the sight of all men. 18 If possible, so far as it depends on you, be at peace with all men. 19 Never take your own revenge, beloved, but leave room for the wrath of God, for it is written, “Vengeance is Mine, I will repay,” says the Lord. 20 “But if your enemy is hungry, feed him, and if he is thirsty, give him a drink; for in so doing you will heap burning coals upon his head.” 21 Do not be overcome by evil, but overcome evil with good.
NAS (New American Standard)