Maybe the boy did kill his father. Didn't hear the scream, did run out in a panic, calmed down later and came back for the knife, risking being caught by the police.
Võib-olla poiss tõesti tappis oma isa, ei kuulnud karjet, jooksis paanikas välja, rahunes hiljem maha ja tuli oma noale järgi, riskides sellega, et politsei ta kinni võtab.