I love @blackkatmagic’s fics!
I very much so think that the bijuu are more like sentient forces of nature? Like, literal walking masses of natural energy–imagine if hurricanes and such were alive and also grumpy. They’re not (before, anyway) actively and maliciously trying to kill numerous humans, just that the humans are so small and insignificant that it’s harder NOT to kill them.
So I think with Mito it was a last act of desperation–whereas with Hashirama it was “oh, we can make the world a better place (for humans) if we cage these walking natural disasters.” Well meaning, but limited.
I do wonder, though, if he was at all aware of the sentience of the bijuu. If he thought of them as giant animals and also natural disasters then to him it’d be like putting down a rabid dog, maybe.