Fredo is a tragic person, but Michael is the most saddest. Its all about American greed.
Fredo was never meant for the Mob life, too kind, maybe autistic/gay. All he wanted to do was to help the family, but he lacked the smarts or courage. His downfall was trying to prove himself, as he didn't know Ola was planning a hit on Mike and lying about not knowing.
At most he should have been kicked out of the family. Instead Michael was too far gone, as he went against his "This is not who I am" when instead he is much worse.
He didn't have to shoot Sollazzo/McCluskey, but he wanted to out of revenge.
He didn't have to kill Moe Green (which in turn got Fredo killed, exposed Michael to the Senate with Frank Pentangeli by accident), but wanted to because of Greed
He didn't have to kill Fredo, but did because he viewed the Mob as family, and forgot what family was. He thought he was smart and found out that he fukked up everything by his own hand. The last scene in Godfather 3 said it all, a broken man dying by a stroke in a chair with NO family.