I’m pro author don’t question my decisions
But seriously he can still be calm and collected but as a cold blooded murderer (against the WG not civilians). Kuzan has claimed countless lives in the government’s name so it really takes a lot for a person like him to maintain a somewhat good perspective on the world, so if at some point the WG would pressure him through his loved ones e.g., to the point where he feels the weight of the atrocities he committed throughout his life, someone like him would be the most prone to a mental breakdown.
He was calm and collected back then considering he was an admiral, and that helps him a lot if he doesn’t want to become a terminator like Sakazuki, but now that urge to control himself is gone, and if the faction he served for so long is now trying to erase him, he going rogue.