Because an enemy of the family could approach him again in the future, just as Roth did. Because he was so jealous of Michael, he might betray his brother again, especially if someone promised Fredo that it would be something good for him and good for the Family.
Michael exiled him from the Family. He was only allowed cursory contact with his mother. He was banned from the Corleone hotels. It took Connie's entreaty to get Michael to give Fredo even a brief respite bewteen their mother's death and Fredo's murder. So, what is it that someone expected to obtain from Fredo in exchange for a promise of something good?
There are a number of possibilities.
For example, he could expose the inner workings of the compound or he could provide information to Micheal's enemies - such as the fact that Senator Gearry 'belongs' to him. Not to mention how much he already knows about Micheal, factors that could be used against him, such as the murders he has committed etc.
Its ridiculous to assert that theres absolutely no way Fredo could have hurt Micheal if he was allowed to live. Where theres a will, theres a way.
