I didn't follow McCain's political career very closely until he ran for President in the 2000 primaries, but from what little I recall he started out as a relatively sensible, realist voice of reason on foreign policy. In the late 1990's, he fell under the wing of the neoconservatives, and has been parroting Bill Kristol's foreign policy talking points ever since.
That accounts for his obsession with Assad - for PNAC, removing Assad and other secular dictators is a higher priority than fighting actual Islamist terrorists.
I believe you might be giving Kristol more credit (or blame) than he deserves. And I wouldn't identify McCain as a "neocon"; instead, I'd contend he's just an ambitious "opportunist".
McCain was all in favor of a full-scale war in Somalia -- even before "Blackhawk Down" (1993). Further, he was all aflutter about intervening in the Bosnian War (1992-95).
It wasn't until the Kosovo War (1998-99) that Cristol seemed to gain his ear. But, as you can see, McVain was already a certified warmonger.
Then, there is the Soros influence. McPain's favorite charity is his very own Reform Institute -- a non-partisan, not-for-profit think tank ...and, since 2000, an extension of Senator John McCain's political ambitions. Soros funded it.
At bottom, McVain is Soros' man -- not Bill Kristol's. And when Soros wants conflict an chaos...Senator John Sidney McCain flies to the Middle East or North Africa or the Balkans...and helps foment it.