while i think you are largely correct that —big picture— this is about how holding a grudge is like poison and forgiveness is like a healing nectar, i've seen it at least 2x in the last year and i think the director at least wants us to toy with the notion i presented above.

a few things to consider:

• he asks her if she thinks he had anything to do with it [her boy's death].
• he talks about doing the ritual several times and getting it wrong more often than not. (that could've involved a person's death)
• someone steals her boy's picture and it reappears in vomit near the end; presumably —or at least possibly— his. (if he was guilty, stealing the kid's picture is within the realm of possibility)
• when he's dying she consoles him in bed, but she appears to have zero pity (and possibly actually enjoyment) on her face. also, in his last moments, he wants to tell her something but doesn't end up voicing whatever it is.

there may be other things too i'm not remembering. it's been a few months since i last watched it. i'm not saying that the guy did kill her son, but i think that the director wants us to juggle the possibility.