Coming back to the original topic, I don't think it is as easy as giving Wyll a default decision, that he can fall back on so he can make his own choice.
If you kill Karlach, I think Wyll has a compelling story that is similar to Astarion and Gale's in that he believes he needs the devilish powers to be a hero but at the same time realises he cannot be that hero as long as he is tied to the hells. If you talk to him at the party after killing Karlach and receiving Mizora's reward for it, he is in anguish over how many innocents he might have unknowingly killed for Mizora before. The theme of receiving a reward for a wicked deed comes up again in Act 2, when you can get an excellent rapier for freeing a devil, a thing that stands against all that Wyll wants the Blade to be. His dialogue after Elminster visits the party at the beginning of act 2 foreshadows the choice he has to make:
"A shame my first brush with the famed Elminster couldn't be a tad more optimistic. Listen I might invoke the Triad from time to time, appeal to Helm, but I'm no man of faith, not like Gale. I don't know what drives a man to consider his own death among countless others to be an appropriate exchange for his goddess's forgiveness, to me it all sounds like nonsense. The faith that matters most is that which you hold in yourself, in the ones that most matter to you. Big bomb be damned Gail's got everything he needs to defeat the Absolute already: Talent nerve and powerful allies at his side. I hope he'll come to see that."
Wyll is of course not willing to die for Mystra's forgiveness, but Mystra's forgiveness does not matter to him. It is his father, that he is willing to throw his life and soul away for. And it is the ideals that he lists, that he does not and did not fully trust in. If he had, he would not have accepted Mizora's deal to become the singular tragic hero to save the Gate at the cost of his own freedom, but would have humbly alarmed the city. (The whole thing about devil deals in the game is that they seem like the only option, but if you have hope you can find another way - like Hope who did just that.)
So when the time comes to choose between his father and his soul, in order to give a compelling conclusion to his narrative, a discussion of theses topics would be needed in order to sway Wyll to one side or the other and instil him with faith in his own ideals.
This gets more complicated when you add Karlach to the mix, because with her in the party, all of this gets reduced down to Mizora being a meanie and all the nuance and character development for Wyll gets lost.
As for Wyll being a moraliser, when asked about his opinion on matters, he usually falls back to morality tales (e.g. after meeting Raphael), quotes his father or makes general statements of what would be the right thing to do.
