Good news, Karlach fans: a Baldur's Gate 3 player has found a way to kill her for Wyll's sake without actually killing her

gamesradar.com:

A Baldur's Gate 3 player has found a sneaky way to obtain the Infernal Robe for Wyll without killing Karlach, at least not permanently. 

As reported by Dexerto, and spotted on Reddit, there is actually a way to keep both Wyll, Mizora, and Karlach happy. As the post reveals, there are a few steps players can take to fulfill Wyll's desire to kill Karlach and keep him human, without actually killing the companion. First of all, you've got to kill Karlach (stay with me now) without Wyll in your party and without speaking to her or her Paladins. The post's author explains that they did this with a disguise "to not risk her recognizing me afterward." 

Your next step is to go to camp and tell Wyll that Karlach is dead. Once this is done, the player suggests you return to Karlach's body and "raise her as far as possible", so she doesn't realize who it is meddling with her. After this, head back to camp and add Wyll to your party, and then have a long rest. As usual, Mizora will then appear and give Wyll the  Infernal Robe. Finally, with Wyll out of your party, you can return to Karlach and recruit her.

It's not clear if a bug caused this player to complete these steps in this order, but it's worth a try if you can't bear the thought of killing fan-favorite Karlach or for Wyll to go without his new robe. In fact, the player has said that a number of interactions that happened after the ordeal almost indicate that this is an intentional part of the game. 

For starters, the second time the player approached Karlach, she asked if they came in peace - which is fair considering the last person to talk to her literally killed her. The player then explains that they had the dialogue option to ask Mizora why she wants Karlach killed