It’s crystal clear by that point that just killing Tav and party and taking the prism is Plan B, considering they weren’t killed on sight at multiple points upon arriving. As is revealed much later,
spoilerwith the prism out of her control, she’s in a race against Orpheus and the stakes couldn’t be higher for her. If she loses, her empire and likely her existence are forfeit. Killing everyone puts her back at square one, and she may not have time for that. Once provoked, she uses a Wish on the party (hell of a way to go out). Obviously, she’s either already tried that as soon as she lost the prism, or she dares not risk it because Wish is notorious for backfiring. Monkey’s Paw scenarios and all that.
Or maybe Wish was just something fun they wanted to include. 🤷♀️