The book was pretty explicit on why he went. He figured it was better to know the trap and prepare for it, rather than risk trying to expose it prematurely to the Landsraad and being exiled.
He didn't expect the attack to be so large. He knew the emperor was involved and he knew the Sardaukar were going to join the Harkonnen forces, but he figured the Spacing Guild's costs would be prohibitive. He assumed that the Atreides forces along with Fremen would be sufficient.
And of course he didn't know Dr. Yueh's conditioning would be compromised.
The plan was to defeat the trap and parade it before the Landsraad.