Any creature (monsters, beasts, npc, player character etc) gets one action, one bonus action, one reaction until the start of its next turn (movement is not any of those). A nilbog can use its reaction to prevent it from dying but a single, individual nilbog has only one reaction until the start of its next turn. If you reduce it to zero hitpoints after it has already used its reaction (for any reason; opportunity attacks for example also use a reaction) then it will not be able to use its ability again so it will die.
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