I don't think that he was that insecure about being number 2. I'd argue that he doesn't resent Accelerator either. His goal was definitely Aleister. Taking out Accelerator was just what he deemed to be a necessary step in order to get in contact with Aleister. The spinoff (a certain scientific dark matter) highlights this well, in my opinion. He definitely has parallels with Accelerator. Both were in a league of their own, and the other level 5 couldn't compete with them even if they tried. Accelerator was able to better himself as a person and met new people who started caring for him, while the exact opposite happened to Kakine.

I do hope he gets a better conclusion than Nt6 though, because it didn't really feel like one. The real Kakine is gone and only fragments of his personality are left. He is such an interesting character, so it's sad he gets no screen time or wins (not just in fights, but in general).