I agree with everything that comes after this statement.
Obama has been given the authority to kill along the officially legislated channels of his office. So, there is the distinction: Dzokhar Tsarnaev had no authority to do what he did. What he did was also clearly immoral.
President Obama has the authority. What he's doing is questionably immoral. It's also not the only thing he does that affects people in a material way good or bad.
This is the relativity of moral decisions and the only way out is to try to define morality in absolute terms, which has resulted in many religions and philosophical works. The purest (i.e. simplest) form of a moral framework is that of Jainism - it promotes full restrictions on all forms of violence except when necessary for survival (e.g. a farmer killing a farm animal to feed his family).
This may seem like common sense, but it helps that it's defined in a simple form. This is why we don't agree with Tsarnaev or terrorist groups - what they do is not necessary for survival, it serves other purposes. This is why we sometimes just as easily don't agree with president Obama's choices. It has nothing to do with "proper channels", "authority" or any of those words.
It is really no harder to manipulate or indoctrinate someone with a high IQ. It's not a measure of resistance to the former.
Of course, a perfectly reasonable question is "how many people does it take to give someone the right to kill?". If 66 million is enough, is 50 million? 10 million? How about a hundred thousand? Which countries, therefore, have leaders with the moral backing to do so, and which are too small.
The idea that popular support makes murder acceptable is ludicrous in and of itself, if you doubt this look at our depictions of Hitler.