Moses gave God's commandments to the Israelites, but Lord Jesus gave to his disciples the beatitudes. The beatitudes are not orders or imperatives, they are suggestions to lead a new life, a life according to the freedom of the spirit. Jesus does not tell us much about these blessings that he exposes, but then he expands on a matter of topics that explains better the meaning of the blessed life.
Jesus talks about the importance of mercy, the love for enemies, but he also tells us something very interesting, we must constantly watch our thoughts: "Anyone who looks at a woman lustfully has already committed adultery with her in his heart" Matthew 5:28. The error or the sin, as well as the assertiveness in our actions, come from how we think, in other words, righteousness and sin come from the heart of man.
About sin the apostle James explains: "But each person is tempted when they are dragged away by their own evil desire and enticed. Then, after desire has conceived, it gives birth to sin; and sin, when it is full-grown, gives birth to death" James 1:14-15. Lust tells us Jesus is the origin of adultery, and with adultery comes murder. Lust is one of the capital sins in man and in many cases implies the death of our relationship with the divine because it gives rise to mortal sins that are the most serious sins; and this is what Jesus says when he continues his explanation of this sin.
Lust as sin is comparable to greed, in fact, among the commandments of Moses there is one referring to both lust and greed: "You shall not covet your neighbor's house. You shall not covet your neighbor's wife, or his male or female servant, hi ox or donkey, or anything that belongs to your neighbor" Exodus 20:17. The Bible, for example, tells us many extraordinary deeds about King David, but when David sinned, he did it out of lust, as the Bible recounts in 2 Samuel 11, because Bathsheba, Uriah's wife, was very beautiful.