So the big mystery in the story is that we know Hanako killed Tsukasa, but killing someone is so outside of what we know of his character and doesn't make any sense.
We know he killed him with the knife he uses to fight, but other than that, we don't get any more details. Until The Red House Arc, where we learn that Hanako was supposed to die before his 4th birthday, but made a miraculous recovery.
In the same chapter it's revealed that the reason for this is because Tsukasa had wished for Hanako's recovery, and threw himself into the mysterious pit in exchange.
It's pretty ambigious what happens to Tsukasa after this, we know he mysteriously comes back after awhile, but isn't the same person. His mother knows this and calls on exorcists to help bring her "real son" back, but to no avail. Tsukasa ages to 13 with Hanako until he is murdered by Hanako himself.
There are a bunch of current theories as to why Hanako would want to kill Tsukasa, from the theories on sexual abuse to the bullying.
For this theory I want to preface, Tsukasa dies when he falls into the pit. He willingly exchanged his own life so his brother could live. The Tsukasa we see afterwards is The Red House, or the pit, just using his body.
The Tsukasa that Kou and Nene meet in The Red House is the ghost of the real one, hence why the ghost isn't 13 but instead the same age when he fell in.
My theory is that Hanako didn't "kill" Tsukasa in the way that we've been led to believe so far. In the most recent chapter we get concrete proof that Hanako killed himself with the knife he uses.
In the end Tsukasa's sacrifice is in vain when Hanako uses the knife on himself. In a sense, he does end up murdering Tsukasa, only realizing the truth of what he did when becoming a ghost to atone.
What about the family suicide?
I really don't have a good explanation for this to be honest, even when not using this theory. I think that it's possible that their parents killed themselves and Red House Tsukasa, but Hanako either escaped or was spared, and Red House Tsukasa simply emerged once again after disappearing.
Why did Hanako kill himself?
Even ignoring the family suicide, it's easy to come up with reasons. The former theories on abuse from Tsukasa are still valid, and the bullying doesn't even need to come from Tsukasa, it could have just been his classmates.
What about The Red House?
The Red House is definitely one of, if not the main antagonist of the story. I'm not sure if it will ever be fully explained, as there isn't really a reason as to why a supernatural would want to mess up families for fun. I think it's much more interesting to see it as some inexplanable Lovecraftian horror being rather than an explaned with a backstory.
This is my theory for now. I'd like to go on later about the connections between Hanako and Tsukasa and The Red House, but other than wish making and exchanges there isn't a lot to talk about as of yet.
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