TB, I've never bought into the Michael thought of as weak reasoning. Tessio did not think that Michael was weak; he thought that Michael's decision to not let him and Clememza "build up our regimes" left the Corleones understrength. Tessio betrayed Michael simply because he thought that, under the circumstances, Barzini's victory was inevitable. When Michael murdered Fredo, he was (and had been for years) king of the underworld. I don't know how anyone in that world would think him weak. He wasn't paranoid; he was realistic and prudent. As he told Tom, our people are businessmen. Their loyalty is based on that. Thus, when the Corleone business becomes unprofitable for them, their loyalty is affected. Michael recognized that just as anyone in that business should.

It's not just a question of Michael's motivation. To murder Fredo, Michael used his son and his sister. That's the apogee of avarice. As you state, Michael was reprehensible.