About the "plunge", JKR said in an interview with Melissa and Emerson that it was gradual (granted, she was referring to why people start to FEAR him, and not even taking his name... but if they feared him, of course it was because he was evil, right?). What I'm trying to say is that, it was no plunge into evil that Voldemort took -- it was gradual. He just went from bad to worse.