Theory as to why Cas couldn't heal the security guard. Other than angst related plot movement obviously.
We know that Jack, as an Nephil sired from an Archangel is stronger than angels, stronger in fact than Lucifer. Archangels are the most powerful supernatural creatures behind Chuck, Amara, the Empty & Death. We have seen snippets of their abilites over the seasons, probably some of the mosr impressive was the almost effortless reality-warping by Gabriel. So we can expect Jack to grow into powers of this range. But I think the most important reference to the power of an Archangel is an offhand comment by Castiel in season 4.18
Not limited, unrestrained, fixed, entire, irrevocable.
I believe this alludes to the fact that, being wholly different creatures from angels; Archangels channel the pure primordial energy of creation like Chuck. An angel, would be unable to reverse or modify in any way the power of an Archangel. Whether Jack's power cancels out Cas' or merely interferes with it, Cas would be unable to revive the guard.
Jack still has little control of his powers, it seems to be an innate reaction to stimulus and only slightly altered by his intent. It's easy to see how Jack could project a desire to stop the Ghoul which explodes as a wave of undirected, unfocused power. Even if he wanted to kill the Ghoul, he doesn't have the ability yes to target him. It took a large amount of effort for him to redirect the bullet which was aimed at Sam in the earlier episode.
So whether Jack wanted to stop or kill, his will essentially altered reality. Nothing short of another Archangel or Chuck could intere with this, presumably.