Well, the explanations derived so far are rather poor, due to Larian's actions... Zariel simply wants revenge on the Hell Riders for abandoning her, the same goes for one of her champions, Karlach.
As with the return of Minsk, Volo etc. pp. there is again a strange chain of coincidental events as a reason. Coincidentally, both live after 100 years, coincidentally in the same environment, coincidentally completely unharmed, coincidentally barely aged. Too many coincidences at once. Let's see who else we meet "coincidentally".
Similar now with Karlach and Zevlor. Both are on Zariel's hit list, but coincidentally both are close by as an encounter for us. Coincidentally, Paladins are also ready to join Zariel, coincidentally near Karlach, which Zariel coincidentally knew about (but not Zevlor...) That's one honestly strikingly strange difference, namely why Zariel or her assigned Paladins didn't find any traces of Zevlor and the other Hell Riders like Tilses and Tiefling civilians, who inevitably leave traces in a larger group, but could easily track down a lone wolf like Karlach. Sure, you could now claim that the others are in the Druid Grove and no one knows it because it's hidden. But I'm pretty sure if Kagha had spotted the Paladins, and the Druids were supposed to, she would have asked the Paladins either for support to fight the Gobbos or after the search for Halsin or for safe escort for the Tieflings to get rid of them. In general, I can't imagine that the Druids have no idea who is currently living in the area - well, the main thing is that they kept meticulous records of the Sharites *cough*.
I am curious how Larian's final implementation will be and whether the inconsistencies I have noticed will be invalidated.
Coincidentally you bump into a great deal of the cast of BG1 in BG2.
