User blog comment:Caramellson/Try Honesty?/@comment-1786741-20120913001421/@comment-24174486-20120914153602

"I still don't get the reasoning behind why he lets you choose Destroy, but simply kills everyone if you decide not to choose."

Shepard's arrival before the Catalyst and the completion of the Crucible convinced the Catalyst that the Reapers' cycles will not work, but it cannot end the cycles without Shepard's intervention. It wants to clean up its previous solution more than it wants to find a new, lasting solution.