And he was right, unlike Nina who instinctively got carried away by her anger and those blue flames in part because of her young age and in part because whatever caused that change on the three sisters, it seemed to have affected the youngest of the sisters the most.
Or Lina who wanted to vent her anger on those guys for targeting Nina, the oldest of three sisters had a much more complicated objective, she was going to remind the Blazehearts why they shouldn't mess with her sisters, while communicating the same message to the Firehearts for the first time as well.
And for that she needed a "witness", someone to live in order to tell the tale, and that was the woman from the Fireheart, since she was the only one among the ones facing her, whom letting live would have some value… also even with her calm and composed self, she couldn't bring herself to let the ones that personally sullied the home where the precious memories of their mother lied, live.