Well, actually, he became a double agent when he found out Lily was targeted, and he started teaching apparently about two months before the Potters were killed.
Originally Posted by leahsm2
However, I agree that fresh out of Hogwarts Snape is a lot different than jaded spy who lost the love of his life Snape.
I think something you're not considering which bears STRONG consideration is this: based on his birthdate of January 1960, his first year at Hogwarts would have been September 1971. He would have left ("graduated") Hogwarts in June 1978. "Snape's Worst Memory", his fifth year, would have happened in June 1976, and Severus started teaching in September 1981, meaning that some of the students who saw Severus being routinely humiliated by MWPP would have then become Snape's students, and of those who didn't witness it, they'd have heard it from those other students who did see it, or their family members who did. I imagine it would have made teaching kind of difficult.
But you have to admit, both his opening Potions and his opening Defence speeches were pretty poetic. The guy's a serious academic and a bookwork that put Hermione Granger to shame.
Remember, too, he took the position at Hogwarts under the Dark Lord's orders. So he probably put a great deal of thought into that opening Potions speech, and by the time the tradition had been established, he'd thought a lot about his Defence speech, too.