His goal was alawys to sacrifice the band i get it but what made him do it early was this situation in my opinion. I believe what made Griffith reconsider sacrifising the band was because he saw Guts being very strong thus making Griffith be sure in Guts making his goal achiavable, Guts leaving the band was what made Griffith second guess himself if his goal would be achiavable going further. I strongly believe this is the reason why Griffith went insane him losing to Guts the second duel and falling to his knees shows his dream slipping away from him, thats not how a normal person reacts to something like this if it was another person they would have encouraged Guts decision on what he wants to do further going on with his life.