I think both actions, the murders and the destruction, are designed to sully us all, to drag us down to their level of blind hatred. The reason why the destruction of historical artefacts is more effective at achieving this is, perhaps, because while we feel some distant sympathy towards "some foreigners that I don't know" being murdered, we feel personally insulted by what we regard (rightly) as our collective, human historical heritage being destroyed. We feel this affects us personally - I know I feel I do, anyway. It feels like a much more personal and intimate insult.
Ironically, the fact that this is such an effective way to get people upset (and thereby polarise them) guarantees that IS, the Taliban, and other extremist groups that depend on global polarisation will continue to destroy priceless historical artefacts everywhere they find them. They depend on the cycle of getting remote people to get emotionally enraged, getting those same people to mistreat muslims local to them, thereby causing more muslims around the world to feel disenfranchised by western society, thereby making ISIS's recruitment work easier.
As one article pointed out after the Charlie Hebdo shooting, the major issue that ISIS has with the west is that the vast majority of the muslims here have zero interest in what ISIS is doing. By letting ourselves be provoked into making generalist statements about whole population (e.g. someone who reads this, gets pissed off, and then goes and votes on tougher laws against Islam in their country), we do ISIS's work.
I suggest responding to this with sadness rather than anger.