Quoting from http://constitution.findlaw.com/article3/annotation24.html : Thus, the Framers adopted two of the three formulations and the phraseology of the English Statute of Treason enacted in 1350, 1285 but they conspicuously omitted the phrase defining as treason the ''compass[ing] or imagin[ing] the death of our lord the King,'' 1286 under which most of the English law of ''constructive treason'' had been developed. 1287 Beyond limiting the power of Congress to define treason, 1288 the clause also prescribes limitations upon Congress' ability to make proof of the offense easy to establish 1289 and its ability to define punishment. 1290.
Thus, I don't see how shooting the president (even 'for the good of the country') leads to a treason charge.
That link also gives a summary of the few Supreme Court cases regarding treason charges. There isn't much, in part because most of the offenses you might come up with are already covered by other laws. For example, 18 USC Sec. 1751 specifically covers "Presidential and presidential staff assassination, kidnapping, and assault."
Actually, no. Treason - as a formal charge - can only exists in a time of formally declared war. Even in the depth of the Cold War, we didn't charge turncoats spying for the Soviets with treason since technically, we were never at war with the USSR. Espionage, yes, treason no.
What he is not, however, is treasonous. He didn't betray his country. He did his country (and many others, including mine) a huge favour by changing his allegiance from government to country - he did the right thing. It'd be nice if those working in government never had to do that, but it'd be foolish to thing they won't, and downright wrong to think they've done a terrible thing if they do.
If you believe he committed treason then you're conflating the country with its government. They are not the same thing at all.