They played too much with life.
No matter how desperate their reasoning to do it was... when you push life like that... it pushes back, and life always wins when it pushes and only death comes from it.
The Vatican's treatment of exorcists as expendable pawns is why they are on the losing end.
They are convinced of this "God's divine will" bullcrap and treat their very limited special soldier with stupid abandon.
Only now I catch that those two scientists were Bak's parents... this does not make this easier.