Possible Metaphor (Spoilers)

The movie is about abandoning the old and the dying. The devil is trying to make everyone desert the old man, because, like the nurse put it, "the worst thing in the world is for a person to die alone", and "a soul needs love to keep it safe".

This devil is no external entity, it lives inside us. The priest hints at this - "He's not out there. He's already here". We see various characters succumb to the devil within - the wife due to the emotional toil of being the sole caretaker for so long, the son due to the demands of raising his own family, the nurse who is practically an outsider and can't help being "blind" to the suffering, the neighbour who wasn't really there.

The significance of all these characters killing themselves is the movie telling us that they had an active role in abandoning the old man, and thus became dead to him.

PS - The only loose ends I see is the priest and the neighbour both sharing a memory of the lead girl killing herself a long time ago.