House of the Dragon director and Eve Best break down Rhaenys' actions in episode 9

gamesradar.com:

House of the Dragon – and the Game of Thrones universe in general – is known for its acts of brutality. However, the most recent installment of the series showed a surprising act of mercy. 

Warning: House of the Dragon episode 9 spoilers ahead!

After Ser Erryk helps Rhaenys (Eve Best) escape from her quarters in the Red Keep where Alicent is keeping her prisoner until she swears allegiance to Aegon, Rhaenys sneaks into the dragon pit where Aegon's coronation is taking place. 

As the people of Kings Landing cheer for their new king, Rhaenys slips below into the depths of the dragon pit – below the boards, so to speak – and makes an explosive entrance in her armor on the back of Meleys. As the dragon roars into the faces of Alicent, Aegon, and the rest of the greens, Rhaenys has made her stance very clear: she stands with Rhaenyra. 

However, if Rhaenys had simply said "dracarys" (the High Valyrian command for dragons to breathe fire), she could have taken out the greens altogether and possibly even avoided the looming civil war.

When asked why, when given the opportunity to burn the opposition to a crisp, Rhaenys chooses not to, writer Sara Hess told The Hollywood Reporter (opens in new tab): "I think she just can’t do it. It’s not her war. The fight is between these two sides and she’s kind of not in it. She doesn’t feel like she’s the one to come in and do that. But you’re right. If she had just incinerated everybody, it’s game over, Rhaenyra wins and we’re done here. But the cost is huge. I also feel like that moment, she looks Alicent in the eye and Alicent walks in front of her kid to shield him. It’s one mother to another. Rhaenys is angry, but in her previous scene with Alicent, she respected her, even if she