The prosecution will have to comb through your email history and prove that your intention was to enable criminal actions. If you build a network to help people escape tracking by their ISP and it just so happens that others abuse it for illegal purposes, then that's not really your fault.

It's not exactly a knife, but German weapons law does have an intent-based loophole: If you want to carry pepper spray to defend yourself against rapists, you need a lesser weapon's license. If you want to carry pepper spray to defend yourself against animals, you don't need any license. If you get attacked by a rapist, and it just so happens you have anti-animal pepper spray, then of course you can use it for self-defense.