Here is where Etri is saying coercion results in mitigation. The "You cannot excuse his actions thus” quote implies that control would result in no personal responsibility for Thomas. This I think Etri distinguishes between control and coercion.
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Possibly, but I also think there is a bit of politics involved as well. It's like even though Harry was able to prove Morgan was innocent, it didn't matter because of appearances Morgan had to be sacrificed. I don't think Etri could explain it in such a way that it would satisfy his people, and this was the best he could come up with. I think what Etri is saying is, yes, you can explain
Thomas' actions thus, even excuse them, but the scales have to be balanced to satisfy my people. So Thomas is allowed to live, but not without pain, because he took an innocent life, so they took his son.. A life for a life, without death.