The best theory I've read is that Jesse gets kidnapped by Lydia's cohorts and is forced to cook for them. The finale sees Walt attempting to rescue Jesse in one final redemptive act.
I did wonder if Skyler and the kids were a part of this rescue, but you're right, it could just be Jesse.
I think Walt's reasons for going into the business at the beginning were sympathetic, he didn't want to leave his family with a huge amount of debt.
That was just the rationale. Walt was sick of his ordinary life and needed some excitement.