[SPOILER] I think the writing for how we react in movies needs to be improved.

[Theodric]

Asahi was ultimately reponsible for the sexual, emotional and physical abuse that Yotsuyu suffered throughout her life. From a narrative perspective, nobody had a greater claim to vengeance than her when it comes to Asahi. So it was a very satisfying conclusion to the arc - and much more solid than the Warrior of Light putting him down.

The Warrior of Light does not need to personally end the lives of every antagonist or villain that enters the plot. Now, if your concern is that the Warrior of Light should have intervened and prevented Asahi from shooting Yotsuyu...bear in mind that the Warrior of Light was likely physically and emotionally exhausted about an intense battle. That easily justifies the delayed reaction...as well as accounting for genuine surprise that Asahi would return after seemingly fleeing earlier.

It's also very likely that Yotsuyu was already dying at that point...and even if she was not, she was much too damaged and broken to ever live a normal life no matter how much she or others may have wished for 'Tsuyu' to prevail. On some level, she seemed to expect Asahi's treachery - as evidenced by her stating that she saved the last of her strength for him.

[Vaiolette]

nobody had a greater claim to vengeance than her when it comes to Asahi. So it was a very satisfying conclusion to the arc - and much more solid than the Warrior of Light putting him down.

Now, if your concern is that the Warrior of Light should have intervened and prevented Asahi from shooting Yotsuyu...bear in mind that the Warrior of Light was likely physically and emotionally exhausted about an intense battle.
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I understand that! However, if that were the case, and we were exhausted, the scene did NOT show it. My point is that while Yotsuyu deserves her vengeance, we should've never been written in a way to just watch someone be slaughtered like a dog a few meters from us~

I mean, we've bested much, MUCH stronger enemies before, so it is very unlikely that we'd be so exhausted to do that. Regardless, I do not disagree with you, but I hope you understand what I mean when I say showing us standing impotently like that is just bad.