Do あなた think Akito killed this bird for no reason または what?

Hm, not sure. I'll have to figure out which chapter that was in.

I don't think she would kill a bird without a reason, though. She seemed to enjoy birds- always having one in her room, she lets them hang on her pointer finger, etc. If it was near the end of the manga, I think it could be symbolic- with the curse as well as her mental state began to crumble, she could no longer enjoy または trust the world like she once did, and even her 愛 of birds was completely strained- and like a lot of things she loves, she hurts. I think there was a dead bird that Arisa found right after Akito stabbed Kureno and walked to Shigure's house- maybe it was like that, Akito hurt Kureno, whom she really cared for, and then hurt the bird, another thing she usually cared for.

But that's speculation. I'll have to 検索 around the chapters to know for sure.