While America debates whether to intervene in Syria, what is often missing is a discussion of the sectarian nature of the conflict. The secularism of Assad's Ba'ath party appeals to Syria's minorities, Assad's staunchest supporters, and angers the more conservative elements in the Sunni Arab community. When the Muslim Brotherhood revolted in the 1980's they chanted, "Christians to Beirut, Alawites to the graveyard." The present conflict has resulted in large measure from underlying tensions in Syrian society that boiled over following Assad's brutal crackdown on protesters. Assad is without a doubt a bad guy and no friend of America, but this conflict has become larger than simply him. Intervening in Syria on the side of the rebels won't bring Syria closer to a just outcome but will tip the scales in a long-running sectarian feud that America has no stake in.
The Syrian Ba'ath party is not a sectarian party, but it drew its most enthusiastic and loyal support from the poor and historically discriminated-against Muslims Sects. The party promoted a combination of socialism and pan-Arab nationalism which appealed to these groups, but angered more traditional Sunni Arabs. Exacerbating this dislike, Assad's father and most of the elites were Alawites (an offshoot of Shia Islam) who practiced favoritism toward members of their own sect. Despite this nepotism, Assad's regime won the support of many other Syrians. Assad's government was unabashedly secular and granted rights to women and Christians, leading many Syrians to see Assad as the best available option (as John McCain found out).
The nature of the rebels is the other element. There are many different rebel groups fighting in Syria, many of them Islamist and quite radical. The Islamist groups have been attacking Christian and Alawite villages, and have executed an Alawite cleric. They have also increasingly been clashing with the more moderate "Free Syrian Army," along with various Kurdish militias. While few Syrians have expressed much affection for Assad, they fear the unknown. The Islamists are the most effective fighters among the rebels, and enjoy support of fellow travelers from around the world; they could easily end up defeating the more moderate forces in the Free Syrian Army.
John McCain is correct in noting that Assad is a murderous tyrant, but who will replace him? It isn't at all clear that any replacement will be better for Syria, or America. Furthermore, a just resolution to Syria's underlying sectarian conflict requires that the legitimate interests of all parties be taken into account, and picking sides doesn't aid that quest. Tragic as it is, bombing Syria won't help.