Belkar and the Oracles Prophecy...

[NerfTW]

He also said he had a memory charm around his valley, and didn't bat an eyelash at simply banishing Roy.
If he really WAS all-knowing/all-seeing enough to be able to foresee his own deaths, he would also be able to see the potential for getting royally screwed over by any ghosts who come back to get him after their foretold deaths.
Actually come to think of it, if he truly was infallible, wouldn't he try to be NICER to some customers just to prevent them from coming back with weapons? Or disintegrate spells?

He never said Roy wouldn't remember anything after being banished, just that he was banishing him. Maybe he wanted Roy to know about Belkar's mark being activated, knowing that Belkar wouldn't remember the clever ruse the Oracle pulled.

How do we know he doesn't have defenses against ghosts coming after him? In fact, we know exactly the opposite, that he does know when a ghost will show up and makes sure he has a rod of banishment handy. Just because he took the time to make fun of Roy doesn't mean he didn't know he was coming.

The reason he's not nice is because A) They won't remember after leaving the valley that he was rude, or for that matter that they killed him. Roy just happened to be there with Belkar. He still doesn't remember the previous two meetings.
B) We already know he knows about and plans for any deaths that may occur from customers.