Has somebody an idea how to get an assassin if the rattling thieve is already death?
Can I get a assassin dagger somewhere else?
or
has somebody a save game with the assassin dagger or shortly before which I could get

Don't think it will be possible to become an assassin if the Rattling is already dead and you didn't get the assassin's blade when he died. Could try hacking a saved game inventory, but you'll need the code for the assassin's dagger. (I don't happen to know it )

Wyv