Desire turns into deception, and deception turns into disobedience. And disobedience turns into death. The way James puts here in verse 15, when lust or evil desires are conceived, it has a baby. It gives birth to sin. Then the grandbaby comes.
Scripture says that when sin becomes full grown, it gives birth to death.
This is the geneology of sin. When Adam and Eve sinned, they died. They didn t drop dead physically, but they lost fellowship with God. And spiritual death is much more tragic than physical death. The key is stopping this process before any sin takes place.