に

狂三を救ってみせると、真那を救ってみせると言いながら、何もできはしなかった。
狂三に、真那に重傷を負わせ、折紙や十香、学校の皆さえ巻き込んだ挙げ句、狂三の力を封印することは叶わなかったのだ。

Hi. What is the function of the に in red? Shouldn't it be は because 狂三 is the subject of 負わせ?
Thank you.

Isn't this the narrator (士道?) expressing their regret for how they weren't able to save 狂三 (or 真那) despite their words, but instead ended up hurting (directly or indirectly) the both of them and getting all the others involved?

The structure of the second sentence is parallel to:
狂三を救ってみせると、真那を救ってみせると言いながら
and implies
狂三に（重傷を負わせ）、真那に重傷を負わせ

i.e. "Even while saying I'd save 狂三, saying I'd save 真那, I ended up causing them both to be seriously injured..."

If it were は making 狂三 the topic of the second part, it would mean, "Even while saying I'd save 狂三, saying I'd save 真那, 狂三 caused 真那 to be seriously injured..." which wouldn't make sense as an expression of the narrator's regret.

(Obviously, I don't know the actual content of the story, so I don't know exactly how 狂三 and 真那 were hurt, but clearly the narrator feels responsible.)

Thank you, benten-san. Though according to the story, it's 狂三 who seriously injured 真那. So I think it should be は. 狂三 was injured too, but not that seriously.

Probably what the author wanted to write is 狂三に、真那に重傷を負わさせ/負わせるようにさせ/負わせるようにしてしまい. 士道 thinks he is the causer of 狂三's action, thus, the first に indicates the causee. He feels responsibility to 狂三 (and other people).

Thank you, Toritoribe-san.