I wasted half my life looking for a one horned Tiefling hoping that Anders and Karlach had their signals crossed and that everything could be cleaned up between them amiably. There are any number of ways to satisfy this quest reasonably and these writers chose the absolute least convincing manner of resolving it
You could heal Karlach, bring her to Anders and force them to admit neither really wants to kill the other.
You could give Anders the Soul Coin and help him satisfy his debt to who-ever-the-fuck he made his deal with,
You could lie without evidence (hard deception)
You could lie with evidence (head off any female teifling and soft deception)
You could talk him out of it (hard persuasion)
Paladin specific Come to Jesus speech (soft persuasion)
Any of those would be more satisfying than the quests as is
