Naive. There was no other word for it. Even Kii, a kid, could see that she was being naive. 
Did she think this was a game? That this was some moment for her to show off? 

[i]'All we do were D-C rank missions...'[/i]
[i]'Usually then when I spy I'm serious...'[/i]
[i]'You highly underestimate the main child of the remaining Aburame Clan...'[/i]

No. This was all the argument of a spoiled child that was happy to receive some special gift that she didn't even know how to use. If she couldn't take a D-Rank mission seriously then she couldn't be expected to take an S-Rank one any more serious. She lacked the practical experience in this field, no matter how gifted of a Aburame that she was. The same was said for Kii and for Ryou. They were all Genin. They were all kids. Did she not see this? Clearly, Shira wasn't understanding the implications of what she'd been asked. She thought this was just some moment for her to show off her skills. Her words and her reaction was all too arrogant...Then she even thought to tell Kii to leave, which honestly hurt his feelings. He'd been looking out for her. She may have taken it as an insult, but in the end he'd been saying the things he had said for her protection. He hadn't said anything that was meant to purposely hurt her, but to point out that this situation was way over their heads. Yet, she told him she was fine without him then compared him to some village exploding freak...

For what? Kii didn't understand what she was saying. He didn't see how what she was saying made sense. 
Yet, he had to respect that it was her choice, as a Jinchuriki, and her privilege to decide what she wished to do. This collective offer was for the Jinchuriki in the end and Kii had barely any right to even be here...So, in the end, his opinion was washed away and he was swept under the table.

"Fine..." Kii would say, relaxing. He'd unclench his fist and sigh, looking away from the General and from Shira, "But i'm not leaving," he said low, almost mumbling. "There's no way I'm going to let you do something this dangerous without me." Kii was going to stick to that. Even if she didn't want him around he didn't want her to do something irrefutable and get herself killed or worse, caught. He could swallow his own pride and bite his own tongue if she wanted him too. "I am sorry..." Kii would add, looking over at the general a moment. Kii wasn't going to dignify the apology with a bow. Instead, he'd just take a few steps back, feeling the cool wall through his mantle. 

Still...Even playing the defeated card, Kii didn't lose his reservations. 
This mission was stupid and pointless and would end horrifically. She was being ignorantly optimistic if she figured random people who hardly knew each other could join hands and work together to accomplish such a badly planned mission like this...But, well, it was that trait that he did like about Shira. She was hopeful, positive and bright. So bright that he couldn't see how she could say half the things she said or think the way she did. Kii didn't trust these Jinchuriki, he didn't trust Konoha and, most importantly, he didn't trust this General that would send a bunch of Genin into S-Class enemy territory just for a bit of information. 

Sighing and looking away, Kii decided that it would be better if he just didn't say anything else for the duration of this meeting. He'd gotten himself scolded by Ryou and Shira already. No point in making this any worse on himself.