Here are suggestions for Kivan and Branwen. If someone who actually has any feelings on the characters wants to edit them, that's fine, since I just opened up their DLGs and made some shit up.
Upon Kivan seeing Tazok
Kivan: Tazok! Do you recognize me? You will pay the price for the life of Deheran.
Tazok: Recognise you? I kill many. Why would I remember their faces? But if I missed you last time, I give you second chance. YOU DIE TODAY INSTEAD!
(Tazok turns hostile.)
If Tazok flees
Kivan: Flee, coward, but I will track you down. You cannot hide from my wrath.
Upon Tazok's death:
Kivan: The beast has fallen, and she is avenged. May his foul hide suffer eternal torment at the hands of his victims!
Upon Branwen seeing Tranzig
Tranzig: I know you! Finally got yaself freed, huh? A pity. I'd say ya looked better in stone.
Branwen: Taunt me as you wish, for your fate is already sealed. By Tempus, I shall strike you down!
(Tranzig turns hostile.)
As Tranzig begs for mercy:
Branwen: I shall grant you mercy, Tranzig. The same mercy you granted the merchants I strove to defend.
(Tazok should react as if the player refused mercy.)
BG1NPC's Yeslick/Rieltar is actually pretty good, but unfortunately Yeslick's tone has been replaced by Generic Dwarf so it's not really usable without rewriting the lines.