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]])
Here "heart" is a metonym for a person's inner being or mind. AT: "out of the inner being, come evil thoughts" or "out of the mind, come evil thoughts" (See: [[rc://en/ta/man/translate/figs-metonomy]])