He's on the borderline.
He got mixed up in some bad shit with some bad people. Killed some bad people out of necessity. Decided he was done with it. Got out, and is trying to stay out.
Todd, on the other hand, is evil personified.
How is Walt not evil?
I mean, a little kid was killed in front of him because of something illegal they were doing. The next day he was whistling a nice little tune, and wasn't bothered at all by what happened. That's pretty devoid of emotion/empathy if you ask me. That's not too far from Todd.
But ignoring Todd (because I think it defeats the purpose comparing these two people) - Walt still had plenty of chances to get out. To make the right decision that didn't involve people dying. He chose not to. While you might argue for the sake of survival (ie. not going to jail to rot) - he chose to do what he had to. Okay. But it still doesn't excuse Walt for what he's done, and it certainly doesn't make him just someone that "got in over his head and killed bad people out of necessity". He killed Mike purely because Mike wouldn't respect him. He had nothing to gain from Mike's death, and that was not out of necessity.
And hell, he had plenty of chances to walk away. Where he wasn't in a position of having to choose to sink or swim. Where his survival was NOT on the line. And yet he chose to continue doing what he was doing, because of his pride/ego.
So at the point that Walt makes the decision to keep cooking when he doesn't have to, you can't even excuse it as "he got in over his head, and killed bad people out of necessity".