Spoiler!
I don't know why Chase killed Sydney, the only thing I can think of is either it was an accident or the Echo entity made him do it!
As for the treasure hunt, fans have a lot of opinions! Mine is that TJ was possessed by possibly Sydney and made the letters, but that doesn't explain why the letters appeared old.
Spoiler!
Sidney kinda wanted to die, bc Micah said that he dreamed with Chase killing him on the lake but he was "grateful about it". My guess is that the Echo entity was tormenting him because Sidney technically killed his dad, and then he was acting weird and maybe tried to kill TJ under the influence of Echo and Chase got nuts.
Spoiler Talk below-
From what I gathered, in Flynn's route, we see the accidental death Sydney did to his father, giving context to the rumors Chase, TJ, and Carl know in TJ's route in their flashback scene: the rumor of him killing his father. So, a kid that bullied you and your friends now having talk that he killed his own dad stacked on top of all that?
This...did not look good for Sydney's reputation at the time along with how much of a dick he acted to the group in general with his roughhousing and bullying. So Chase was already wary of Sydney from the get-go once that secret was out.
In TJ's route, The day Chase saw Sydney mess with TJ at the beach with pulling him about in the water, he assumed the worst with intentions and He finally got fed up with Sydney's antics, thus leading to Chase killing him and the possession taking over him from that point onwards, changing him for the manipulator we see controlling TJ now in the present.
Now, whether or not it was from pure hatred or being controlled by the (socket man?) spirit when it appeared at the beach, I'm unclear of. Everyone disliked Sydney but Flynn. So I wouldn't doubt that Chase would go that far.
I see, makes sense.
Spoiler.
From what I remember of that flashback scene TJ really did appear to be in distress and Sydney was aware of that and thought is was funny!
It is said that Chase was mean to TJ too, but he didn't ever go that far!
Now that doesn't justify Chase killing Sydney, but beating him up would have been fine.
Spoilers
Well, it's not the first time Chase confronted Sydney about this! Recall when he did just that at the railroad tracks, pushing him about to show how it feels. When he fought with Sydney at the beach, he was screaming about Chase trying to kill him and the situation just escalated from there.
