Ah yes, the hospital. I know where he means; the Robert Macgavillary Memorial Hospital. Peter Kemp owns it; if there is an authority on supernatural activity in London, it's Peter, though he's not particularly supernatural himself. He's a long story. The upshot is that he is one of the most knowledgable, trustworthy, and to some extent notorious (in the best way) people we know.

It's entirely possible that Jason wasn't born that way, yes; particularly if there was some dramatic change. I have known good people to become demonic because a trigger made them emotionally unstable and vulnerable for a time. Then, the spirit of a dead person, an evil person, would take advantage of that state to merge with them, and fuel their negativity; anger, hate, jealousy, grief, greed. Jason may have been the victim of such a spirit.

It's a lot to take in. But I am very much relieved to know that this hasn't shaken your confidence in your friends.