Ability to Decipher Hashem's Will

Bilam seems to obey Hashem at every step in Parshas Balak. He refuses the first messengers and goes only when told he may. So why do Chazal and Rashi read every word of his in the harshest light, and call him Bilam HaRasha? The answer starts with a contrast with Avraham and with a small detail in how Hashem phrased two different commands, one to each of them. A father who finally says "fine, go" to a pleading son is a picture of how a person hears what he wants to hear, and what that costs.