Who is Responsible? The Developer? The User?

Anonymous Coward II asks: "I am working on a paper for a computer ethics course, and need to answer the following question: Who must be held responsible: The person that develops a software that will (or can) be used to illegal ends (like to break into a computer system, to illegaly monitor other users, a virus, etc), or the person that use it afterward? I'd like to know what Slashdot users think, and what is the answer according to the law." Software is a tool, just like any other, so when things go wrong I think this then boils down to a question of personal responsibility or negligence. What are your opinions?