When polly went to meet Jason she saw him kissing Cheryl and she attacked him. She seems a bit crazy so could she have hit him on the head and he get knocked out.
Her parents who learnt about Polly's plan then find her next to Jason and after Polly explains what she saw and they see what she did they send her away to protect her and they say she never reached Jason. It would explain why they said 'if he is dead I hope he suffered' because they know that Polly might have not killed him but also he might have been. They hope he suffered because he cheated on Polly WITH HIS SISTER and that 'hurt' Polly and if Polly does have some form of condition it could have made it worse, another reason to send her to the sisters.
Jason then could have woken up and met Jughead, Jughead seems to know a lot about what has happened and was there when polly and Jason were discussing meeting by the pond it is made clear. Maybe he went there to go find Polly and Jason, and because Jason has been knocked in the head he might be confused and said more than he meant to, which would explain why Jughead seems to know so much.
Jason could think Polly is still out there and reasons with Jughead to let him go on, and Jughead agrees but secretly stays near Sweetwater River watching him, because he knows Jason's been hit on the head and could get in danger.
Jason was dealing drugs and he desperately needed money maybe he scammed the people he gave drugs to. They were angry and they got somebody to kill him who has links to the serpents and they go to shoot Jason. They shoot twice one of the times they mistake Archie for Jason because they were told he might be with a girl (Polly) and Archie was with Miss Grundy but they see Jason wandering in the distance and realise he matches the description better as they have seen a photo of him. They go after him and for a while, in which time Archie packs up and leaves as he is freaked out by gunshot and does not hear the second.
When the person goes to shoot Jason again Jason thinks he sees polly (the shooter) and then runs up to them, his vision is blurry so he does not see who it really is. He gets shot straight away which is why he is shot in the head if he was running away most likely it would be in the back of the head. Jughead then sees the murderer and Jason but realises that the murderer is linked to the drug dealing, which links to his dad so he agrees not to tell.
The murderer buries the body as Jughead is there but then months later on the night the body is found
Jughead decides to put it in the river so that Cheryl especially knows he is dead and is not waiting for a phone call. He also seems to be writing about what's going on so he could have done this as part of the next bit of his story, and it would explain why he knew the night was 'far from over.' It could also explain why he is so reluctant to let everyone know his dads a serpent, not just for shame but because he is worried they might make some links.
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RIVERDALE THEORIES!
FanfictionBasically a load of theories about who killed Jason Blossom in Riverdale