Here's the problem - If the guy actively is levying war against the United States, or has given aid to an enemy of the United States, it is considered treason, which is punishable by death. However, a charge of treason MUST be proven in a court of law, and every United States citizen is GUARANTEED the right to due process of law.
I am not saying I want this guy hanging around if he is going to attack the US, but it is the process that must be upheld. Not to protect him per se, but to instead protect ALL citizens. If they can 'ignore' the Constitution and Bill of Rights today, who is to say they won't do it again tomorrow?
Why did we perceive Saddam Hussein as such a bad guy? Because he violated many of the human rights that we hold dear. He ordered the execution of his opposition, he imprisoned, tortured, and murdered the people whom he felt were a threat to his government, and he did it all based on nothing more than his own directive. There was no trial to protect these people, and they did not get an opportunity to refute the charges against them.
I am in no way trying to compare Obama to Saddam, just trying to show that the US Constitution exists specifically to protect ALL citizens of the United States.
Odds are that they are not wrong, and this guy is guilty as hell... but WHAT IF they are wrong?