Eileen hadn't meant to speak out loud, but clearly she had.
The threat seemed odd to Eileen at first, would that be worse than killing her? But she didn't want people in Niflheim to know, even if Sal knowing, which it looked like he did, was really the worst thing.
Either way, Eileen had to drop the whip. She let it fall to the ground.
She held some faint hope that if Sal wanted to kill her he'd have done it already. But then again maybe he planned to get what he wanted from Henry and Mitch and then kill her.