I think Harald and Rocks died the exact same way: getting Domi Reversi'd by a God's Knight killing their Wife.
In Rocks' case, I think Harald put him down.
In Harald's case, Loki stabbed Harald while Jarul held Harald's arms back, and the sword went through Jarul's head, causing memory loss.
Garling is the obvious choice, but, Gunko interests me, here.
Gunko killing Teach's Mom Eris, and Loki/Hajrudin's Mom, Ida, leading to Rocks and Harald's death, would give Teach, Loki, and Hajrudin REALLY legit reasons to want to kill Gunko, which Brook will obviously want to stop.
Since Garling is the Champion of God Valley, I still think he kills Eris.
But, I could really see Gunko/Shamrock being the ones to kill Ida.
This would give Loki further reason to despise them, if Shamrock and Gunko snuck into the country 14 years ago, and killed Ida to Domi Reversi Harald, through Imu possessing Gunko.
Shamrock would have been 25 at the time, and this would explain why Shamrock visits Harald's grave: Shamrock met Harald at the Reverie, with Shanks, and liked him, but, was ordered to kill Harald.