The protagonist's mother must have been killed by Childs. If it wasn't for him, he wouldn't have put a red stamp on it, because he didn't know what was going on, and would simply ignore Shin.
Since Childs was the murderer, why didn't the father let the protagonist identify him? Either they are accomplices, or they are afraid of revenge. But getting lost in the rainy night and meeting the murderer is really a coincidence. So the father is an accomplice.
So no matter what the red or black stamps are, they will die because the envelopes given to Childs are poisoned, not the stamps.
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