One of the major themes of "The Godfather 2" is Michael's transition from a kind man to a hardened, business-like criminal. Therefore, his treatment of Fredo by this time is "strictly business". Fredo has to be considered a threat to the Corleones after his betrayal, which is why Michael has him killed.

If you are still unconvinced, remember that this kind of behavior is not new to Michael; in "The Godfather," he kills Connie Corleone's husband after he betrays Michael to Don Barzini, even after he apparently forgives him.

One of the major themes of "The Godfather 2" is Michael's transition from a kind man to a hardened, business-like criminal. Therefore, his treatment of Fredo by this time is "strictly business". Fredo has to be considered a threat to the Corleones after his betrayal, which is why Michael has him killed.

If you are still unconvinced, remember that this kind of behavior is not new to Michael; in "The Godfather," he kills Connie Corleone's husband after he betrays Michael to Don Barzini, even after he apparently forgives him.

EDIT: See the comments below. They make good points, though I'm pretty sure the Carlo is very much a part of the family and both Fredo and Carlo betrayed the family in a similar way.

One of the major themes of "The Godfather 2" is Michael's transition from a kind man to a hardened, business-like criminal. Therefore, his treatment of Fredo by this time is "strictly business". Fredo has to be considered a threat to the Corleones after his betrayal, which is why Michael has him killed.

If you are still unconvinced, remember that this kind of behavior is not new to Michael; in "The Godfather," he kills Connie Corleone's husband after he betrays Michael to Don Barzini, even after he apparently forgives him. Michael knows better than to trust unwisely.

One of the major themes of "The Godfather 2" is Michael's transition from a kind man to a hardened, business-like criminal. Therefore, his treatment of Fredo by this time is "strictly business". Fredo has to be considered a threat to the Corleones after his betrayal, which is why Michael has him killed.

If you are still unconvinced, remember that this kind of behavior is not new to Michael; in "The Godfather," he kills Connie Corleone's husband after he betrays Michael to Don Barzini, even after he apparently forgives him.