Hank knows that Walter was and maybe still is involved in the meth factory. He found that out in the bathroom in Walter's house as he flipped through a poetry book inscribed to Walter from one of the team who had been murdered. Incidentally, Walter was among the killers.
After that happened in the final episode of this season in "Breaking Bad" some of us assumed that Hank, being the dutiful earnest law man, would attempt to take his brother-in-law into custody. Walter would die during a shoot-out or by suicide. We recall the scene of his sitting next to a gun, contemplating deliverance.
Well, there is another scenario. Walter can pick up on the reality that Hank knows. Since he has already murdered so many others and had even attempted to poison a little boy, there is no doubt that he wouldn't kill Hank.
The only reason he might hesitate is that he suspects that Hank already informed his superiors that Walt has been a kingpin in the meth business. In that case, Walt could scoop up the $700,000 Skyler has squirreled away and the family and go where "they," both the law and the gangsters, couldn't get to them. Making those plans and implementing them would generate plenty of dramatic tension since Hank is enmeshed in the White family's lives. Jr. has become quite close to him.