Our world is not very peaceful, the main reason is that there are too many unfair things. How to deal with such things? Using law? Not enough. In fact, the law itself is not very fair, so we need some other means to solve it. In my opinion, assassins may help.
For example, if an official is very corrupt, but the law doesn't justify him, assassin should teach him a lessen, even end his life. This is a good mean to maintain justice.
I admire assassins talked in books in ancient time. They murdered emperors, and they gave the people justice. They are heroes all the time. Our society has greatly developed, but there must be something that will never change. This is why we still need assassins.