Quote 1
[The Misfit:] "I forget what I done, lady. I set there and set there, trying to remember what it was I done and I ain't recalled it to this day. Oncet in a while, I would think it was coming to me, but it never come." (113)
That The Misfit literally can't remember what he did seems unlikely. How could he forget what sent him to prison in the first place? Maybe if he were innocent and falsely accused it would make sense. However, we know from what he does and says in the story that he's far from being either. It's more likely his "not remembering" either indicates that he doesn't feel as if his past actions are wrong or doesn't take them seriously.
Quote 2
"No, lady," The Misfit said while he was buttoning it up, "I found out the crime don't matter. You can do one thing or you can do another, kill a man or take a tire off his car, because sooner or later you're going to forget what it was you done and just be punished for it." (123)
Here again, The Misfit mentions "forgetting," and it doesn't seem like we can take him literally. What really makes The Misfit a bad person is that he doesn't have a sense of guilt. He's not troubled or haunted afterwards by what he does; none of his crimes feels wrong to him. That's why he can speak of forgetting what he's done. Even if he did literally forget his crimes, (perhaps after all he's committed so many he's really forgotten some), this could only happen because they don't affect him at all. They "mattered" so little that it was possible to forget them.
Quote 3
"No pleasure but meanness," [The Misfit] said and his voice had become almost a snarl. (134)
The Misfit commits crimes – killing people and destroying thing – because there's nothing else for him to do. He's not motivated by any desire for gain, and he doesn't believe in the concepts of right and wrong. Destruction seems to be the only thing that gives him pleasure. But you might wonder: given that he recognizes what he does to be "mean," doesn't he have some sense of right and wrong? Could it be that it's just because something is wrong that it gives him pleasure to do it?