Russia doesn’t want to kill ISIS. They want to kill secular opposition to Assad and block the Gulf gas pipeline route to Europe.
It has nothing to do with gas pipelines. Syria has been Russia’s ally/client for over 50 years. The Russians, whatever their flaws, stick with their allies until the last dog dies. Look at Cuba.
And there is no secular opposition to Assad. Whatever there was wouldn’t have lasted two minutes against ISIS. There are assorted jihadists and Assad.
The pipeline won’t happen without ‘regime change’ and ISIS is supposed to facilitate that. So the Russians are motivated crush ISIS and and at least hold the line against Saudi backed al Nusra.