Rewatching the series for about the 4th or 5th time now and I thought about this scene again.
I think by this point, Tui has become almost feral after having been alone in the wilderness for so many months, being stressed out over the birthing procedure/bleeding, having the trauma of Jamie's death and then the extreme pain/trauma/stress of the birth itself.
At this point, I think such a reaction would actually be natural from anyone under the conditions she has had to endure. Almost as if this and the shooting is all she has strength for at this point.
When she later seems dismissive of the baby, that too is a pretty natural reaction from a 13 year old for any responsibility or draw from their own lives.
I think the hissing indicates the extreme stress of the moment...and probably also because she subconsciously knows that Matt was the father of her baby.
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