Here the word "within" describes a person's heart. AT: "come from within a person's heart" or "come from within a person's thoughts" (See: [[rc://en/ta/man/translate/figs-ellipsis]])
"Heart" here is a metonym for "thoughts and intentions." AT: "from one's own natural desires, evil thoughts come" (See: [[rc://en/ta/man/translate/figs-metonomy]])