You are so precious

I don't think that Tagusari brothers killed Haruko. They were too emotional to pull the trigger and too calm when strolling on the seaside. I think they purposedfully missed the shot and the bullet hit an oil barrel or something. If Haruko died, her body should be shown, even though it's just her hair or hand on the ground, and the blood should be pooled much more, not just three drops.

We still have a mystery about who gave Haruko the poisonous plants book (or maybe I forgot about the details, maybe it's Amagi(?)) and maybe the brothers would pursue him.

I think they walked into the police station to make a report about past crime (Haruko's) or tell the truth to Kosei. Although the case is already expired, I think they're still thinking that they had a responsibility to close the case completely. I think they left Haruko at the port, and Haruko just stayed there until sunrise, drowning in her guilt and thinking about what the brothers just did to her-- not killing her as their revenge, but letting her live and hopefully she would repent her sin.

I think the Tagusari brothers did not kill anyone. But, they did manage to release the clog in their head and the flow cleared.