Someone inciting someone to commit a crime is to be punished as if he committed the crime himself in addition to the actual perpetrator. §26 StGB
Murder is punished by lifelong imprisonment. §211 StGB
The attempt of a crime is punishable by the same punishment as the execution of it, or with a reduced sentence. §23 StGB
You can agree to your body being harmed by someone (like in a martial arts tourney), but that doesn't make it legal if the harming of the body is against "Gute Sitten" (~morals) §228 StGB This includes (attempted) murder.
Hiring a hitman is punished by lifelong imprisonment for inciting someone to attempt/commit the crime of murder. Failure in murdering someone makes it an attempt. If your incitement of the (attempted) murder can be proven, it doesn't matter if the hitman never tried to execute the murder or grossly failed - Or even if you failed to hire the hitman: The attempt of inciting someone to murder you is punished as if you had murdered someone.