Deciding if you should kill or save Regalla in Horizon Forbidden West is something you'll need to contemplate late in the story, and initially it isn't obvious what different outcomes are possible following your selection. There are actually three choices that you're presented with once the battle has concluded in the Wings of the Ten quest, though one of these is a mere distraction. That then leaves you with a choice between saving Regalla by saying "There's another battle here," or killing her by selecting "Your life ends here" as your dialogue.
The main difference between letting her live or killing her is that if she lives, she'll accompany you in the final mission of the game during the assault on the Zeniths. It's a largely token gesture, adding more of a visual flourish than altering anything. If you kill her however, you will get her bow - Regalla's Wrath, which might actually be more worth it. You'll get something tangible out of it at least.
Regalla acts as one of the game's major villains, and yet there's something quite sad about things when you discover what exactly has been going on behind the scenes in Horizon Forbidden West. So, with all of that in mind, let's go into a little more detail on the two choices:
If you choose to save Regalla, you'll ask her to join the fight against the Zeniths. Although most people are skeptical about her loyalties, Hekarro states that she's always true to her word.
Regalla agrees to fight because attacking the Zeniths will be a warrior's death and better than being snuffed out in the Arena. She then accompanies your crew back to the base and will join your assault on the Zeniths for the final mission, Singularity.
She then dies in the final assault, getting the warrior's death she wanted.
If you decide to kill Regalla, her story ends there. Kotallo runs her through with his spear before you can so much as move, and she dies mid-sentence. This does two things: firstly she won't appear in the final battle, and when you pass the area she would have died in if she was alive, nothing happens.
Secondly, and more usefully, you get her bow – Regalla's Wrath. This is actually an incredibly powerful sharpshooter bow (think sniper) that can one-shot some small machines. So, while cold blooded murder feels very un-Aloy, it's actually the path that gets you best outcome here.
Wait or go now in the Horizon Forbidden West embassy choice? | Horizon Forbidden West Aether, Poseidon, or Demeter | How to find Poseidon in Horizon Forbidden West | How to follow the orb and find HADES in Horizon Forbidden West | Investigate the battlefield in Horizon Forbidden West Cradle of Echoes | Where to find the source of the signal in Horizon Forbidden West | Horizon Forbidden West Yarra or Drakka choice