I don't think anyone is slutshaming Hannah, or at least the hate for Hannah has nothing to do with her sexual deviance. More that she is manipulative and ruins people's lives. It goes well beyond her cheating. Another thing, probably for the worse, is that we as viewers are largely desensitized to things like murder or killing, and it is more abstract, whereas we have an aversion to lying and manipulation on a closer and more visceral level.

One way to look at it is that the screenwriters did her wrong by not giving her a redemptive arc. The other way is basically just to say this is just a show, if a character is not given a redemptive arc maybe we are supposed to take it as she is just a shitty person.