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.
Yeah, all that went away the the moment he went into business with Fring.
I don't think poisoning Brock is the worst thing he did. He did not do it 'in cold blood', that was a tactical self defense maneuver and one of the few times his tactical self defense maneuver was designed not to be deadly. The murders are far less forgivable than what he did to Brock, and his refusal to let Skylar distance his family from him when he had every intent to put them in the path of violent thugs.
It's obvious Lydia's going to try to force them in some manner back in the game, but I think she's go after Walt and his family long before she went after Jesse. She's way too scared of Walt. Kill Walt and kidnap Jesse, maybe.
Just sayin', Lydia's a coward, and she's a coward who has seen what happens to people when they fuck with Jesse without dealing with Walt first.