Something like that can only make sense within the cultural context of ISIS, because to the rest of us it is just appalling - just as all the other such videos were monstrous and appalling.
If the goal is to scare off the US or other countries from getting involved, then they misunderstand what is happening (US gov't doesn't care about journalists at the best of time, and might use this as a justification anyway). Picking some random journalist and killing him makes about as much difference to Western foreign policy as holding your breath until your face turns blue.
More likely their goal is to boost local support by demonstrating what badasses they are. I don't know if that will work as I am not a local, but it sure doesn't win them any friends abroad.
My condolences to the family. It is a brave soul who becomes a war journalist.