Jack ended up not killing Ben in surgery because of the implant.
Some, most or all of the Others are implanted to ensure obedience. This explains the self contradictory behavior of Others like Juliet. This explains why the Others we see seem so emotionally unstable.
Some of the Others have learned they are implanted, but because of the implant itself, are helpless and cannot revolt.
Juliet has known she's been implanted for a while. This explains her self contradictory and erratic actions. This explains why Juliet wants Jack to kill Ben.