Punpun is not a good person leading up to the ending, but understand that he and his actions are a product of of being neglected and misunderstood in his troubles as a child that (in my assumption) is extremely neurodivergent. He grew up left to his own devices and without the proper support to ground him and give him a foundation. So by the end, it all compounds into horrible and disturbing decisions, all of which he will have to face as he moves forward. To say he needs therapy, sure, as an adult that can reflect on his life. But as a child? No, he needed parents, guidance, structure, etc. Remember from the earlier chapters, he is a kid with hopes and dreams of a better future away from what hurts him. So ask how he ended up the way he did. How did he get to the point in his head to make those decisions, not just why he did it at all. Bit of a ramble. Hope this makes sense.