Well the Real Reason Why Wyll and Karlach dosent Have a Evil Path as companion..
Its becouse Karlach was Shoehorned in the Game in the Last Moment and they Had to Change Wyll Questline to acommodate Her as some Old Players from the Beta told..
I beleave that since they did not Have Time for Making Such a Change so close to Release that this Happen..
another Thing people Says its that Halsin was a NPC.. yu would not Get a Companion from siding with Grove..
i Beleave the Original Plan was to Have Jaheira as companion in Good Path and Minthara as evil.
(but people Pleed for then to add Halsin and thats Happen.)
thats why the Story of this 3 characters are laking so much
but 1 tip.. yu can Have Jaheira if yu go Evil.. she can Join your Party and Minsc too even if yu side with goblins and Get minthara..
Yu just Have to Not Kill the Lady from Last Light.. if yu are the Durge.. Kill her Later, if yu are evil origin or evil Tav.. Dont Side with Marcus.. she will go to your Camp after the Last Light Fall if yu Chose.. and yu can go Nuts and rampage around without her going away. Btw Never let her go to Act 3 alone or she will die in the Curse.