Since ISIL is in about 200 locations throughout Iraq and Syria to be effective Russia would need to turn both countries into nuclear wastelands, and several other countries downwind from Syria and Iraq. If they just lob one nuke at a specific concentration of ISILites that will have little effect on ISIL, but a huge effect on everyone else.
Remember, these people are not merely willing to kill for their cause, they are willing to die. So killing a few ISILites is not going to make them have a rethink about what they're doing.
“The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.” - Bertrand Russell
"When the power of love overcomes the love of power, the world will know peace."
Jimi Hendrix
