That's exactly what the creators' vision was for the show, shdw. Great observation. They took a real risk taking a really really good guy and turning him into a bad guy over the course of the series with (at least according to their interviews) no intention of turning him back.
I stick by my stance that Hank will not be turned. There has to be at least ONE straight shooter (as someone described him earlier) simply too keep Walt honest. If they had planned on turning Hank, I feel like it would've happened way back when, when he was busting up Jesse. They could've had Walt then trying to turn him.
Hank won't turn because WHEN he discovers who Walt really is, there'd be no way Hank could justify siding with him. All of Walt's dealings is what has caused Hank to be hunted down by the cartel brothers, put in a wheelchair and almost permanently paralyzed, accrue thousands and thousands of dollars in hospital bills (you know he's too prideful to "enjoy" Walt and Skylar paying those), whiplashed into a head-on collision with Walt driving, developed an obsession over the blue meth, had a hit put on him and his family, and has the whole DEA telling him he's wrong and crazy.
If he turns, all plausibility this show has, is out the window.