In addition to his other morally questionable actions, we should probably consider his sending off a little girl to what could well be her death. I mean, that's basically the case when he sends Dorothy to kill the Wicked Witch of the West.
The MGM movie has him asking Dorothy and company to bring him the Witch's broomstick, to which the Tin Woodman replies, "We'd have to kill her to get it!" Why he assumes this and whether it's indeed true are never addressed, as Dorothy ends up killing the Witch accidentally.
In the book, though, the Wizard just tells the main characters to straight-up assassinate her. It's certainly possible he thinks they won't even try. Really, sending someone off to do a seemingly impossible task is a pretty typical motif in fairy tales and myths. Take Polydectes telling Perseus to bring back the head of Medusa, for instance. The dispatcher always expects failure, but the hero succeeds anyway.
The Wizard does notice that Dorothy wears the Silver Shoes and bears the kiss of the Good Witch of the North, both of which should provide supernatural protection, so he might be holding out some hope that she'll succeed, even if it's not really conscious hope. He's certainly surprised when the party returns bearing news that the Witch of the West is dead, but he also tells them, "When you came to me, I was willing to promise anything if you would only do away with the other Witch."
Still, there remained the very real possibility that Dorothy (who, after all, is a stranger in Oz) would go up against the Witch and fail miserably, which would have to prey on his conscience, wouldn't it?
Regardless, Dorothy doesn't seem to mind, and when she meets the Wizard again in Dorothy and the Wizard in Oz she bears him no ill will. And when the characters recap Dorothy's first adventure at the beginning of Ozoplaning with the Wizard, the Wizard's sheepish response to this part of the tale is simply, "But, you see--I didn't know any real magic then. And besides, I needed more time."
All I can say is that Oscar is lucky Dorothy and Ozma are so forgiving.