hiten mitsurugi, i understand what your saying. why use a kunai when she can use her flute? she's been using her flute for however long she's been fighting, and she's always won with it.
However i think your taking her statement too strict / line for line. It doesn't mean that her whole time fighting as a ninja, she's never used anything except the summons to kill someone. It means more along the lines that she's used the flute to gain victory, not nessicarily to kill everything. And dude, shika got her mad at some degree.
Even if she hadn't killed anyone with anything except her summons in the past, think of her as a real person. real people dont one day decide, "i'm going to kill everyone with my flute for the rest of my life."
She said the statement about using only her flute in a prideful manner/boasting. Its not some mantra that she follows.
And seriously, if she was a real person, and thought that shika was in a genjutsu, she wouldn't take the damn time to summon the monsters again just so she can say my flute is my only weapon.
Even if she <u>had killed shika with the kunai</u> she'd probably consider her flute her only weapon still. GRANTED, the kunai being used is good for the story, but its also just easier and makes more sense.
Or you can believe that the statement she said is a mantra / a way of life for her that she'll never break, but that would make her character more petty/stupid.
realisticly, she thought shika was stuck and couldn't move. since she's a ninja who uses all the assets given to her, and that she never carries anything except a flute, the kunai was a good oppertunity. you can say it was stupid of her to take it because it was a trap by shika*shrug, then you'd be right she should know shika's smart* but the only real justification i can see for her to use the flute to summon and kill shika is if she was being extremely cautious(suspected something from shika) or she wanted to follow some sacred vow of hers.