As Sigs said, you need to take Tom's finger. There's no way to let Taurus kill him; Taurus always goes after you first (he doesn't like executions to be witnessed).
That chest is only unloced if you take the finger to Luxurius, then tell him Tom's dead. He'll unlock it before you end up fighting him.
Note that taking the secret way in will get you s skill book as an additional reward, whereas taking Tom's finger won't give one.
