There are two main groups opposing Assad in Syria: The pro-democracy rebels, and the Islamist ISIS. The West should be supporting the first group and exterminating the second: ISIS are terrorists.
The war is being fought by several factions: the Syrian government and its allies, a loose alliance of Sunni Arab rebel groups (including the Free Syrian Army), the majority-Kurdish Syrian Democratic Forces (SDF), Salafi jihadist groups (including al-Nusra Front) who cooperate with the Sunni rebel groups, and the Islamic State of Iraq and the Levant (ISIL, better known as ISIS).
Russia is leaving ISIS alone hoping they damage the Coalition forces while they bomb what would be the opposition parties were Syria a democracy, in order to prop up Assad’s regime.
Having said that, once the fighting starts, it is difficult to distinguish between the Freedom Fighters and the Terrorists. The group I have most sympathy with is the Kurds, who have been persecuted minorities in Syria, Iraq and Turkey, and have never had a homeland of their own, but have done the Lion’s share of the fighting against ISIS on the ground. If there were any justice, they would be granted eastern Syria and northern Iraq as a new country of Kurdistan.