Is it being a "poison arrow trap" from the LN or something? Because afaik, all that's mentioned in Ch32 is that she "ends up falling into his trap" and dying (and iirc Gain doesn't use any arrows/darts in his fight with Reito). But that doesn't necessarily mean it's an actual trap, after all Airis also says she also managed to kill him, which if she just got hit by some random trap would be kind of surprising. It seems more likely Airis talking about Gain's "trap" just means him lying in wait for her, they fight, they both die.
But even if it was a poison arrow trap, part of the whole thing is that Airis can't actually tell the future beyond Reito's involvement in an event. So there's no reason to actually think that Reito defusing any traps that might've been lying in wait would've been sufficient to let her kill Gain herself, without just doing that instead anyway.
It's a bit lame that he keeps quiet about it to Bal instead of admitting it, but if he's trying to save her it makes sense why he wouldn't want her anywhere near the situation.