If you do not have the necessary power to take the president, you do not have the authority to decide whether he is corrupt. If you are able to take him in a more civilized way, either through getting him impeached or by having a significant enough force to take the president into your own custody rather than using a sniper's bullet, this would in my mind be acceptable. But if you have to go out on your own, stand by the window on the third story, and take a shot, do you really have the authority to say that the man is evil? It could well be that you are the one who needs to be put down. You cannot assassinate the president.
I think it is the duty of United States citizens to protect the country from corruption. It is clearly stated in the US Constitution. The duty of Americans is to keep the government honest and trustworthy, and when the government goes too far it is time to fix it by any means necessary.