A righteous but gullible whistle-blower must obtain evidence her intimidating boss is a murderous monster before he catches on and takes sadistic steps to silence her for good.

You're welcome. Looking forward to the next one.

Hello Darkhorse, thanks for your insightful feedback. I am going to revise my longline, it seems I have used my intended mid point rather than the inciting incident for my longline. Perhaps this novice realtor discovers her boss's fancy lifestyle is bankrolled using clients money held in escrow so she hires a lawyer to investigate but he catches on to her and hires a hitman to kill her. I will revisit this concept and come back. I'm a beginner at this. Thanks again. Steeldownunder.

Is this a bit like Disturbia when the protagonist suspects that the person is a killer but has no evidence and no one believes them?

Also, I don’t think the word whistleblower fits in this situation, because alerting people to someone being a murderer is just good sense, not a controversial revealing of corporate corruption.
Unless you’re saying that the protag was a whistleblower in the past, so everyone is salty towards her and no one will believe her suspicions that this guy is a murderer. That would be interesting.

If he is a murderer, and he knows that she’s on to him, then wouldn’t he just kill her straight away. Wouldn’t be a very long film. I don’t understand what the sadistic steps are, other than a plot device to draw out an otherwise short conflict. So I guess, maybe the stakes could be different. Like the protag’s friend or sister is dating him, or something, and she’s worried that he will kill the friend/sister. And no one will believe her suspicions so she has to do the dirty work herself.