“securely.” The Hebrew is “in security,” or “in safety.” The idea is that your neighbor is living trustingly beside you, so don’t devise evil against him. That could happen for a number of reasons. A person could become envious or jealous, like Cain who killed Abel, or it could be that a if person was starting to get involved in some ungodly activity he may start to want to get a godly neighbor “out of the way.”
“by you.” The Hebrew is literally “with you,” but we felt “by you” more fully represented the intent of the Hebrew text. Today, “with you” likely means the person was living in the same home, while “by you” means close to you. However, in this context in Proverbs, the person may indeed be living “with you,” in the same house or same tent, but even then he is “by you.” The word “neighbor” in the Hebrew usually refers to someone who is close to you in physical proximity, i.e., “near you” or “next to you.” Thus, in the Hebrew, a “neighbor” could be the person eating close to you in the same house.
Police are quick to point out that a large percentage of the violence that people commit against each other involves people who are closely related in some way: they are of the same family, or they live together or near each other, or they work together. In those situations, we can annoy each other and even plot to harm each other. God knows that, so the Bible warns us about planning evil against the people close to us, knowing that that is a major problem in human societies.