As with all things, read Luther in context, read when he is using satire and rhetoric, and (if you can) read the original German rather than an English translation.
As Catholic scholar Jared Wicks said in his book,
Luther and His Spiritual Legacy: “One needs to be on the lookout for Luther’s rhetorical flights, and to be judicious in discriminating between the substance of his message and the linguistic extremes with which he sometimes made his points.”
The point Luther was making is not to go out and murder or fornicate as much as possible; it was to remember the infinite sacrifice of Christ’s atonement: there is no sin that Christ cannot cover. To say that there is would be to limit the Atonement and limit the power of God.
Perhaps Luther WAS just trying to make a point, but if he was he chose very poor language. Let us compare:
Luther:
Be a sinner, and let your sins be strong, but let your trust in Christ be stronger, and rejoice in Christ who is the victor over sin, death, and the world. We will commit sins while we are here, for this life is not a place where justice resides. We, however, says Peter (2. Peter 3:13) are looking forward to a new heaven and a new earth where justice will reign. It suffices that through God’s glory we have recognized the Lamb who takes away the sin of the world. No sin can separate us from Him, even if we were to kill or commit adultery thousands of times each day.
Note the difference in the language and image between Luther and St Paul. St Paul describes things done
to Christians and how Christ covers and protects them. Luther on the other hand chooses to describe things being done
by “Christians” and claiming that Jesus will cover these as well.
Paul is talking of Christian - Saintly - Strength and perseverence in persecution and sorrow where Luther is talking of Justification in Faith regardless of how sinful we are and continue to be.
Certainly not what a good Christian Parent wants their child to learn
Peace
James