It's quite simple actually, Assad was democratically elected by the majority of Syrians to become Syria's leader, the minority however didn't want this and started a civil war.
Assad is accused ( and probably accurate) of being a Russian puppet, as the Russians want access to the countries vital resources (i.e oil)
The west decided to plan and execute a coup against Assad by supplying the rebels, which led Russia to arm the Syrian government and army to fight back. While both have been fighting each other, ISIS emerged and these terrorists with other terrorist groups ( e.g Al Qaeda) infiltrated and took over the west backed rebels. They want the resources to carry on and extend their holy war or jihad.
The West continues to back the now terrorist rebels to topple Assad, while Assad and Russia are now trying to (unsuccessfully) destroy ISIS and the terrorists.
IT WILL END LIKE AL THE OTHER TIMES THE WEST INTERFERED IN TOPPLING DICTATORS AND REGIMES THAT GO AGAINST WESTERN IDEOLOGY ( E.G AFGHANISTAN, LIBYA, IRAQ ETC.)