Giving Kagha a means to aid the player would reduce the "maniac evil PC" problem because if she had a solution then siding with her would be a choice that would make sense for a self interested, rational, but ultimately amoral person to choose.
As is she offers nothing. You as the player are looking for a way to fix the tadpole problem. She asks you to get the tieflings to leave. Why would you do this unless you just hate tieflings? Generally you wouldn't because there is nothing to be gained for risking your life. But if she could offer something the PC wants while the tieflings don't an evil character has a strong reason to consider her offer and do something horrible for their own gain.
This also enriches the choice to side with the tieflings because it means your good aligned character is turning down a potential way to save their own life because they refuse to do something morally repugnant.
It adds nuance to the choice.
As for the consequences of choosing the evil option of siding with Kagha? I assume how you handle this will influence Act 2 when you run into the tieflings again in Baldur's Gate. Or don't as they're all dead.
By and large that should be how the good vs evil choices are handled. Evil gives immediate, short term gains but cut out options later in the game because of your actions while good choices are crippling in the short term due to your sacrifices but have big pay offs later in the game when you run into people you've helped before.
The "evil" path does this right now and has you betray the Tieflings in order to gain a potential cure / solution to the parasite problem through Minthara and the Absolute. The problem is that this also leads to every single other option becoming completely unviable (Halsin dies or is otherwise out of the picture). Kagha has no solution for you because if you "side with" her, Halsin gets locked out of the grove and helps you anyway.