Ignoring all the legal conventions, I'm going to say the client is more responsible. If all assassins in the world were to suddenly disappear, that doesn't mean the prospective clients don't still want somebody dead. And if there are no assassins to do the job, I'm sure many will find some other way to get it done. They are motivated by emotion, which takes any opportunity it can.
On the other hand, if all clients were to disappear, chances are the assassins won't kill anybody. Money is their motivator. So without anybody willing to give them money, they don't have a reason to bother.
On the other hand, if all clients were to disappear, chances are the assassins won't kill anybody. Money is their motivator. So without anybody willing to give them money, they don't have a reason to bother.