Forum:KB:Why did Vader become evil?

Wondering if anyone has a more in depth explanation as to why he became evil. I know why he turned to the Dark Side, and I understand why he betrayed and hated the Jedi, but I just read Coruscant Nights:Jedi Twilight and in the book Vader is exactly as we remember him from the Original Trilogy, a really evil guy. But about halfway through the book I remembered that it only takes place a couple months after ROTS. I don't get how Vader became so evil so quickly, he was force choking and threatening people throughout the whole book, yet just two months ago he was a young Jedi Knight who just wanted to save his life. Like I said, I know why he turned to the Dark Side, but I just don't understand how his whole persona changed just two months after he got his suit.