Thousands of years ago, Kree experimented on primitive humans to create first Inhuman Warriors. Later, the program was abandoned and some Kree Reapers were put in stasis to forever orbit the solar system, in case the need arose to kill the Inhumans. And, its activation system was put on the Earth itself.
In season 3 of Marvel's Agents of S.H.I.E.L.D., the Hive even activated the Inhuman kill switch on purpose. Although, he had initially said that it was the only weapon which could kill him, he easily managed to kill one Kree Reaper without taking any help from others. Even Daisy was able to kill one Kree Reaper on her own.
Why was Kree's kill switch so weak?