Also, there was the implication that Clarke was under the control of a Keeper. It's possible the Shadows put it on him right after he became President, or the Drakh put one on him after the Shadows left when the Civil War began (when Clarke became a real villain rather than a background one).
That whole "Circle the letters of a repeated sentence to spell out a secret message" thing? That was the real Clarke getting past his Keepers' control (to warn everyone of the self-destruct the Keeper set in motion) before it made him kill himself.
This of course would be another cop-out: Rather than just being some extreme guy who thought his ideas for Earth had to be done, Clarke turns out to be a madman because he was under the control of 2-D bad guys.