Before an act of murder or adultery is committed there has first been the motivations of the person involved. In his or her heart there has been a murderous anger or an adulterous lust. What Jesus says in the Sermon on the Mount is that many people may have the same motivations in their hearts without ever carrying out the external actions. There may be many reasons for not acting upon our motivations, but obviously one of the most common reasons is a fear of the consequences. The laws of all societies make it perilous to commit murder and laws or social pressures of all societies make it costly to commit adultery. Therefore when a person refrains from such actions it may not be because their heart is pure but simply a matter of self-protection. Jesus is saying that where the motivation for not acting on one’s desire is selfish, that person is as unrighteous in God’s eyes as the person who actually commits the crime.
– William Hordern
as quoted by Tchividian, Tullian; One Way Love: Inexhaustible Grace for an Exhausted World; David Cook Publishers; copyright 2013; Kindle Edition; Location 434