Some could argue that by releasing the small prisoners, he would get to take out the big guy, ie Ichigo. Seriously, what was Orihime going to do to him, she and the others were no threat at all. He killed two birds with one stone, he would get a chance to fight Ichigo and still be obeying orders.
Yes, "some" could argue but not the Gotei 13, Kenpachi didnīt only released only Orihime, he released her, Ishida, Chad and the sand dude. And at least Ishida was strong enough to defeat Captains and yet Kenpachi released them all the same. It doesnīt matter how weak or strong they were, he still broke the law therefore making your argument flawed.
Why do you think that Tousen and Komamura stopped him?
Kenpachi broke the law and didnīt even tried to hide it, rather, he said that he was fucking insane and told both of them to go at him.
So we have Kenpachi breaking enemy prisioners and fighting Captains because of his initial felony.
Describing this as anything but insubordination and disregard for law and order is ludicrous. Kenpachi acts almost exactly like Ikkaku and like i said, the only difference here is that he doesnīt hold back (That is, only when he sees that his adversary is strong enough).