Remember, Darski, that a DoW, by itself, does not give you war weariness. There is a list of things that do cause weariness, but just being at war does not.
To review, the biggest cause of weariness by far is losing one of your cities to the enemy. Other significant causes are: being attacked (even if you win), and attacking and losing. Minor causes are: losing a worker or settler, having your improvement bombed or pillaged, or having a unit bombarded to redline.
Notably absent from this list is attacking the enemy and winning. As long as you are the agressor, and you win consistently, you will never get significant weariness.
However, you never get actual war happiness unless the other guy declares on you. That can happen if you tell them to go and they refuse. But the way you described it, you should have gotten the "Are you sure?" message when you first whacked a settler, and then you chose the "Yes, they are scum!" answer. In that case, you don't get war happiness. If you didn't get that dialogue, then you were already at war somehow.