I can understand complaining about pacing issues because this manga likes to time skip a lot but if you really think Charles-Henri just did a 180 for no apparent reason...you must have either skipped through a lot of the manga on autopilot or have the brain of a peanut and should stop commenting on manga comment sections altogether.
Ever since he agreed to inherit the Sanson post, he already sealed his fate. The desensitization he gains after every kill and being absorbed into a system where he can somewhat glorify his deeds in the name of 'justice' would already make it an uphill battle for him maintain his beliefs. What looked like the straw that broke the camel's back for me is when he entered the forbidden room and saw his father's chapel.
By imagining his dad having the same sympathies as himself, he gets lulled into believing that maybe he and his dad are not so different after all. And that is the most cunning trap of them all, making him more susceptible to being the executioner the First Estate wants him to be and convincing him that 'bearing the cross' that the head of the Sanson family must bear is a duty an idealist like him could do while keeping his conscience.