Fredo was done with since the SUPERMAN scene, sayin that Johnny Ola brought him there after saying he didnt know him. If he would have confessed when earlier he probably would have saved him. After the Kiss of Death scene i believe Michael would have killed him no matter what happened. If it seemed that Michael was being nice its only because he wanted info from Fredo. Maybe if Fredo would have betrayed him in the GF1 or GF3 he wouldnt have been killed. But Michael's personality in GF2 was monstrous.

"Well you just do what I tell you to do! Goddamn it! If I had a wartime consiglieri -- a
Sicilian -- I wouldn't be in this shape! Pop had Genco -- look what I got." - Sonny Corleone