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The photo in post 4 matches the one I saw and my turtle ID skills are questionable but to myself it looked like an adult green. I found reports of similar behavior from a green at Akumel (just across the channel) in 2013. In the video on post 9 I understand it was also a mugging, it came up unannounced and snatched it.
Juveniles aren't yet completely herbivores but this doesn't look like a juvenile. Perhaps someone had been keeping it as a pet and hasn't developed naturally? Even the aggressive behaviour is odd, meal choice aside.