Michael wanted Fredo to get in the car with him (and dispatched Tom to find Fredo after Michael returned to Tahoe) to pump him for more info that was relevant to the Tahoe attack and Roth's plan to have Michael killed. Having come so close to death, Michael couldn't afford to let any opportunity go by to find out more about any remaining threats to his life. This became even more critical after he got back to the US, and Tom told him that Roth survived and got out of Cuba.
Would Michael have spared Fredo if Fredo hadn't made that "I was stepped over" speech? Probably not. Michael had a policy of not giving any enemy a pass. The penultimate page of the novel explains it:
After the Great Massacre of 1955, Kay leaves Michael and takes the kids to New Hampshire. Tom goes there to convince her to return. She demands to know why Michael couldn't have forgiven Carlo and Tessio for having betrayed him. Tom replies that they had to be killed because "treachery can't be forgiven....they would always be dangerous....He would be shirking his duty to you and his children, to the whole family, to me and my family, if he let Tessio and Carlo go free. They would have been a danger to us all, all of our lives."