There's couple of details missing from that initial spoiler.
The severed head is split into pieces. Like literally, pieces.
The crew put together the pieces to make the head.
The samurai proclaims anyone he has cut, was done so without shame. (in other words, he's proclaiming he's never cut anyone without honorable cause) and he will cut tens of thousands more to get to his son.
His statements and predicament suggest the samurai is a good guy.
Also, I believe the bird-human Usopp spotted was a bird-woman.
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