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The sheriff beat the suspect again and again until it seemed he might die.
Literal
Sheriff [topic-は] suspect [obj-を] opponent [subject-が] die-complete [nominalizer-のではないか] [quotative-と] seemed [until-まで] many-times beat-down.
A long, grammatically dense sentence. Key pieces: (1) 相手 ('the other party, opponent') refers back to the suspect from the sheriff's perspective. (2) 死んでしまうのではないか is 'might end up dying' — ~てしまう for finality + のではないか for soft speculation. (3) ~と思われる is the passive of ~と思う, giving 'is thought to, seems' in an impersonal frame. (4) ~まで marks the temporal limit. (5) 殴りつける is a compound verb for emphatic beating.