I'd like to know what it means that there was a skull left behind after Suriel killed Mitch Shelly. Because he isn't there anymore. Somehow, being killed caused him to appear close enough to Gotham that he was remanded to Arkham Asylum. Still, he left a corpse behind (part of one, anyway) but doesn't seem to be missing any vital bits in intensive care. Maybe instead of a new soul after every resurrection, he gets a new body? Except he usually pops back up where he fell. Unless her attack severely dispersed his atoms, and they didn't recombine until near Gotham. Which might jibe with Suriel's concern that she had perhaps destroyed his soul.
Frankly, if we're going to buy into the notion of a soul, the idea one can be completely eradicated is a little frightening. Can it be done without destroying the body, in which case you end up with one of those Hollows, like Green Arrow was when Kevin Smith, er, Hal Jordan brought him back? Can the body operate as before without one? Say, an angel destroys your soul, do you go, "Ouch," rub your chest, but continue on your morning search for coffee and crullers? Does having or not having a soul make a difference in terms of taking part in creating a child? I'm guessing a infant's soul does not come about in quite the same way as its genetic code, but in the new DC, who the heck knows? Maybe that's how we get a Ragdoll with no soul.
It could be connected to his being injected with that regenerative compound we saw during the flashback, but I think that's a feint. Certainly it gave Bonnie and Carmen healing factors and considerably greater than human strength and speed. Mitch dies, then comes back to life, hale and hearty, rather than simply healing before he can die. He only has superhuman strength and endurance if that particular resurrection provides him with it. Maybe the compound unlocked something in him, something the weird creature saw which prompted it to give him the injection, but I have my doubts.
Even the alien feels like part of something else. It may have recognized something inside him, but that wouldn't mean it's responsible for his resurrections. We can't be sure he hadn't had resurrections before then. It might have been awhile, he might have been better at hiding them. Maybe the compound is the reason his memories are so shot, and it doesn't have anything to do with his powers. They only had one test on one subject where it showed promise, which is no guarantee it wouldn't have different effects on someone else.
One other thing I find interesting is Suriel simply assuming Mitch's soul would be on its way to Heaven. Certainly he's seemed like a well-meaning guy thus far in the series, but as we've seen, he was a right bastard just a few years ago. We know the other side is interested in his soul as well. Is there any guarantee the good he's done the last few years would outweigh the bad? Of course, Suriel basically caused a plane crash, then shrugged it off claiming it was supposed to happen, so perhaps they're unfazed by Mitch's past actions. If the Spectre and Eclipso are any indication, Heaven is run pretty shoddily. Or stupidly, whichever.