He was gonna betray his comrades, take their amazing talents, it was all very in character of what we've seen from him up until then... and then he suddenly tells Immortal Solider his ENTIRE PLAN, and the Immortal Soldier just completely accepts it and is completely fine with everyone he cares about getting betrayed. Which is not at ALL in character, given that not even 2 chapters ago, he was planning on shooting first and asking questions later simply because of the SUSPICION that the main character might be a spy.
And, for some fucking reason, we then get a flashback to him telling the Musashi girl literally everything about his whole plan. I can only assume that the authors suddenly realized that they had a female main character and they needed her to remain important to the plot, so that's where the random ass flashback where she gets every piece of information came from. And for some reason, SHE doesn't care about the possibility of her entire friend group getting betrayed either. It's just like, what the fuck? Where is this confidence in this random person coming from? You've known him for a grand total of MAYBE a few weeks.