I always thought it was likely Karel did eventually kill Guy which may have resulted in a downward spiral to him ending up reforming and becoming "saint of swords".
Karel: ...Some things you will realize only after you have lost something else. However, by the time my foolishness had left me...it was already too late.
The above quote states that obviously he lost someone or something which really screwed him up. But he has only just found out Karla has died, whether they had grown apart too much and he hadnt had chance to be a good brother to her properly. Possibly he regrets not spending enough time with her. Since he now states he is only here for his family, and he states to Bartre that he must of thought he was a terrible brother when he was younger.
Also Lucius and Karel implies that he is completly worn out from all of the fighting,
Lucius: ...You look tired.
Karel: ...What?
Lucius: Your face seems pallid, drawn, and worn. It looks unhealthy
Maybe it all just got too much, the human body eventually gives in.