f the anti-Assad rebels are radical Islam and anti-western, why would the western powers be pursuing a defamation campaign against Assad?
Because the West is, by and large, oblivious to what's really going on. Western intelligence agencies still seem more geared towards the Cold War than towards really understanding the Middle East. They see Assad as an ally of Russia and somewhat anti-Israeli and label him an enemy, Then they see a rebellion against him and make the false assumption that "The enemy of my enemy is my friend," ignoring the very strong radical Islamist bent of these (and other Arab Spring) rebels. They mistake a pro-traditionalist Islamic social/religious movement for a pro-democratic political movement. As in Iraq and Afghanistan, the CIA and other Western intelligence agencies have no fucking clue what's really going on among these strange foreign people, and don't seem particularly motivated to try to understand (or capable of it).
And all that is exacerbated by a Western press who love a good "David vs. Goliath" freedom-fighter story, and seem to look at everything in terms of "good guys vs. bad guys." So the press in the West have cast the Arab Spring rebels as the "good guys" in their fictional narrative, instead of realizing what's going on in REALITY. That's why they and their governments are so caught off guard by events like the Libyan embassy attacks, or other attacks on Westerners in these "liberated" countries.