Yes, state-sponsored assassinations are justified, if they are absolutely necessary to protect the United States people, or to end humanitarian problems. It would be a positive thing if the United States could assassinate the leaders of North Korea, for example. That would be a way to end humanitarian suffering in that country, and more lives would be saved.
Assassination means killing people. Killing people, for any kind of reason, is not good at all. Say that you want your country to be safe, you can give a life imprisonment sentence instead, so that he/she will not harm the country or its citizens. Even Buddhism gave a severe punishment for killing.
No, state sponsored assassinations are never justified. Every living human being deserves the right to a fair trial, no matter what his or her crimes may or may not have been. An assassination is a death sentence without a trial. It is thus a violation of human rights to use such.
Assassinations make a country look bad, rightly so. Whenever possible the state should strive to capture people rather than to kill them. Not only can the reasons for an assassination be wrong, the state can be killing the wrong person, one totally innocent of the crime they're thought to have admitted. Assassinations make a country look cruel and barbaric to others.