It might seem like a plot hole at first that Amleth chose to hide his identity rather than seek out his kinsmen on Orkney. But given he spent most of his exile believing Fjölnir was a king it makes sense that he would not feel safe there.
From a Viking perspective, taking his heart wasn't the worse thing Amleth did to Thorir. By killing him while he is asleep and unarmed in his bed, Amleth denies him entry to Valhalla.
What happens to Fjölnir's freed slaves? As a character remarks earlier on in the movie, even if a slave escapes, they'll still be in a desolate region of Iceland with limited means to leave. Even if they have the necessary supplies to escape (which, considering that the village was on fire when Amleth left to duel Fjölnir, is uncertain), they'd still be prey for other Viking raiders or slavers.
