I don't know, I thought the transition ran smoothly enough. I mean, his decline started even in episode 1, when he went from "hey, I could do good with this thing," to "I could rule the world as a God!" I struggled a lot within the show because I liked Light; I mean they developed him as the main character. But now and then he would do something absolutely douchebaggish, like sending the wife of the cop he killed to go commit suicide, and I would get a bit closer to accepting that he had to die.
As for what happened after the whole memory trick, we always knew he had murderous intentions for innocent characters that we liked (for instance, L). I would think that after purposefully losing his memory and getting it back in time exactly according to plan, he might get a bit of a big head. It's fully possible that this may lead him to make much bigger (and consequently more evil) moves, maybe even getting a bit less careful with it all.
Beyond that, after that whole thing the stakes got higher. Light killed L (which was pretty much the last straw for me: you kill L, you die), and even let his own father die according to plan: and he was happy. He didn't care who he killed, all he could see was his goal. He had become completely numb in the process of achieving his goal.
I do agree that he seemed a bit careless, which is a bit strange. But I attributed it to his descent into insanity and evil throughout the course of the show, a transition I could see pretty well.
Side note: I'm not trying to disprove you or anything, I just enjoy literary discussions like this.
