My guess is the turtles are not guilty. But I don't have another perpetrator. Snappers will usually suck in prey, not unlike an LMB. So they grab bite sized critters. Red ears etc won't cause any trouble.

All of your turtles will help clean up carrion though.

Do you have bluegill, bass etc in the pond? If so I would expect them to be earlier indicators of a water quality issue. Both carp and catfish are relatively bulletproof. So water quality is unlikely.

No new predators? River otters or even cormorants seen lurking in the shadows?

4 acre pond 32 ft deep within East Texas (Livingston) timber ranch. Filled (to the top of an almost finished dam) by Hurricane Harvey 9/17. Stocked with FHM, CNBG, RES 10/17. Added 35lbs RSC 3/18. 400 N LMB fingerlings 6/18