Even though he knows how hard the road back is, Hal still didn't do all of those things consciously. Sinestro on the other hand did everything on his own accord, without being "possessed" or going through trauma after failing to save his hometown. Not only that but he was the one who directed Parallax towards Hal in the first place. So for Hal to still be this forgiving of him, he must still see him as his old mentor to a degree, rather than as the guy who's done every awful thing there is to do under the sun.