I usually agree but in this case I kind of get it, his team had seen the way he went after the secret demon queen and the demon lord also knew. Like he exposed himself when yelling at the secret demon queen and the blonde guard (forgot his name) wants to ask him questions, I think like chapter 170. Sure he could have lied but with the demon lord knowing, there isnt really a point.
Well, I think is just what MC said: he told his team the truth because he wanted "peace of mind". After all, when you treasure someone you don't particularly like to keep secrets between you.
In MCs case it probably was a mix between that and not wanting to spend his life wondering if they would hate him, should they know he was their former enemy.

I never understand the point of telling people your a transmigrator. Or, at least I didn’t until I saw another character struggle with existentialism due to what being a transmigrate entails. I still probably wouldn’t tell anyone though, but that’s just me.