It may or may not have been an accident. As far as Russia is concerned, the so called moderate rebels the US supports against
Assad are also their enemy just as is ISIS.  Russia supports Assad and anyone who is fighting against Assad is the enemy who will
have to be dealt with even after ISIS is driven back into the dark hole from where they came.  It may not be possible to destroy ISIS and their ilk
completely but reducing their relevancy will be enough.  ISIS after all is just the more recent resurgence of the islamic fundamentalist drive to take over the world which has expressed itself in bloody violence several times in the past, kind of like the tide lapping away at the beach,
eroding it a little at a time which is how Islam has spread itself all over the world.  Unless Islam undergoes a reformation of sorts, it will
always rise up and attempt world domination by force.