[Dante of the Inferno]

Originally Posted by Guernsey

One thing bothers me, how did Satoshi get Hinamizawa Syndrome in the first place?

Satoshi has been in a L2 state ever since Satoko has needed his constant protection from their aunt and uncle. He reached L3 when he confessed to Rena about hearing the footsteps. He hit L4 shortly after killing his aunt. He hit L5 after he started hallucinating in Dr. Irie's car and seeing his aunt everywhere.

Edit: It should be noted that the mental/physical(?) stress of defending Satoko and himself from their aunt and uncle is the trigger for Satoshi's increasing L-state.