“and that soul is guilty.” The Hebrew can also be understood to say something such as, “and that soul realizes their guilt,” and some versions translate the verse that way.
“unfaithfully, yes, unfaithfully.” The Hebrew text contains the figure of speech polyptoton, doubling “unfaithfully” for emphasis.
[For more on the figure polyptoton and the way it is translated, see commentary on Genesis 2:16.]
[See figure of speech “polyptoton.”]
“unfaithfully against Yahweh.” In this case, as we can see from the context, one person has sinned against another, but that sin is said to be “against Yahweh” because such sin is rebellion against God.