ISIS and other violent Islamist groups don't hide their motives for deliberately massacring innocent people. Here[1] is a statement by ISIS after the November 2015 Paris attacks. It's about what you'd expect. They are certain that our culture corrupts the pious and leads people to Hell. From their point of view, there's no action worse than this. It's far worse than killing, as that just sends the pious to heaven.

There's also the "wedge" factor: These attacks sew mistrust between all Muslims and the rest of the society, hopefully (from ISIS's perspective) alienating many nonviolent Muslims and driving them to become more radical. But this is just a side benefit to these attacks. ISIS would be committing these atrocities even if they didn't contribute to anti-Muslim sentiment.

Note that both of these reasons are completely unrelated to avenging the west's actions in the middle east. ISIS would be doing this (and in fact is doing this) to civilians from countries which have never been involved in past oppression.

The key point you have to understand is that Jihadists really take their religion seriously. They're like the Branch Davidians or the Peoples Temple... if those cults had grown to millions of adherents.

1. https://ent.siteintelgroup.com/Statements/is-claims-paris-at...

At least, that's how I understand ISIS explains it themselves in their promotional material, haven't actually read those myself.

The more people hate muslims, the more they feel like outcast. The more muslims feel like outcasts, the easier it is to make them extremists and recruit them for their army.