If a murderer was going around killing people, would it be justified for you to drop a bomb on his house if it meant killing innocents who live near him? Killing innocents is unjustified when ISIS does it, and unjustified when we do it. End of story.
The air strikes thus far have been very targeted. I haven't even seen any reports of mass civilian casualties. Do you have a link? Ultimately it may not be possible for there to not be any civilian casualties, but the civilian casualties are very low with the kind of targeted air strikes we're doing. President Obama has received quite a bit of criticism for not expanding the air strikes.