Why fight ISIS?

[Anomalocaris]

Before dealing with them I'd rather find out why, otherwise considered sane people from foreign countries are flying over to join their mindless cruelty.

Because a sizeable portion of any population naturally have escapist power fantasies involving violence and cruelties.   When section of the population become disaffected by the prevailing social, economic, political conditions, the portion of those with escapist power fantasies involving violence and cruelties will Be tempted to respond to opportunities to act out such fantasies.   The temptation can be magnified if the opportunity for violence and cruelty can be protrayed as being in service of something grander than the petty concerns of ordinary moralities and human sympathies.