According to Diplomacy Screen,
- Kongo has met so far : England, Arabia
- Arabia has met : Kongo, Sumeria
I'm befriended with Sumeria (2 cities).
It took me 35 turns to capture 3 of Kongos 5 cities. (Detached 2 Swordsmen, 3 Crossbow) Even when I give back the 3 cities (size 1,2,3), he won't cede the 4th city (size 4) to me in a peace deal. I think I will take all 5 cities and see how Diplomacy develops with the other civs.
I also approached Mekka with a crossbow. Interestingly, Saladin sent me a settler as welcome present, maybe to lure me into range of his archer and catapult. He should have known that I was coming since he pulled the settler back in the previous turn.
Edit :
After investing some money into Diplomatic Delegations (75 Gold each), I noticed -2 Warmonger Points so far with every civ, even when they do not have met Kongo or Arabia. (Medieval)
Edit 2 :
It is still medieval era but more than 50 turns since DoW. Warmonger points are still at -1 with most civs.
I conquered 4 of 5 cities from Kongo (incl. capital) and he will now give anything in a peace deal.
If I return the Capital and have him cede 3 cities (size 1, 2, 3), I will receive -6 Warmonger Points with most civs.
If I have him cede 4 cities (size 1, 2, 3, 7), I will receive -12 Warmonger Points with most civs and additionally a permanent -18 with Kongo for occupying the capital.
If I take the last city and eliminate Kongo, I will receive -24 Warmonger Points with most civs.
Taking the single arabian city results in -17 additional Warmonger Points with most civs.
So eliminating both with decay in between brought me to -39 and caused most civs to denounce me. Reasonable deals are no longer possible and unreasonable demands occur frequently.
Rome was an exception and did not display warmonger points for me.
Elimination also means that you do not get the cash the civ has accumulated and which it would give in a peace deal, in my case about 1.000 Gold each ...