Of course the outrage is justified. It's not hypocritical either, but it's narrowly focused. We don't exactly accept comparable brutalities like those committed by Western powers. However we're so used to them, we omit them from any moral balance sheet.
The brutalities are the oldest of old news: when the US conducts air strikes, it knows with certainty that civilians will be killed. It conducts them anyway. As result, living innocents get their arms torn off, their eyes blasted away, their legs severed. The US also knows some of these people aren't adults. They are children. (These acts of the Obama régime are no worse than many acts of many other régimes, North and South, East and West, throughout recent history.)
Again, to be clear: the US acts with certain knowledge of the atrocious mutilations and killings that will result from its actions. If I blow up a house when I know I will shred or squash to death several innocent people, it doesn't really matter if I just 'intended' or 'wanted' to kill a someone I considered a very bad person. I am judged a murderer and held responsible for these crimes. That's a judgment often and rightly visited on Israel, with appropriate repugnance. But I never, I mean never, see that holding-a-dead-rat-by-the-tail disgust at the US among those who vent their outrage at Foley's killers.
IS commits in-your-face atrocities. The US commits off-stage atrocities. The US victims suffer exactly the same agonies, are just as human, just as innocent, and end up just as maimed or dead. They gush blood; their guts spill out into the dust. If you cannot maintain ongoing repugnance at US actions - and I'm not saying you should - don't get all shocked by IS actions. It's not a good idea. Better to understand that no, not everyone is outraged at Foley's death, quite the contrary, and they are no more morally skewed than you are. Unlike you, they do keep in mind what the US has done off-stage, and how its lovely decent Western allies hardly make a peep about it, and about how the lovely sensitive Western public is equally unperturbed.
To bear that in mind is maybe an important step in promoting a world in which James Foley doesn't meet a brutal death at brutal hands. To reduce brutality probably requires an honest understanding of what it is.