Because it was a trick. You are being tricked by the voice that you are responsible for minimizing the harm they choose to inflict on the three people in the room, if that threat is even real in the first place.

So to choose to murder anyone or kill yourself is a ridiculous position and most wouldnt take it. The voice will have to be the murderer here if thats what they want.

Or you can believe the voice without question, and kill one of the people based on some arbitrary metric you come up with on the spot to justify you choosing to kill someone.

So in this case, people are believing the lie, then choosing "the lesser of two evils" based on some arbitrary metric like "which ones better for the economy, since they both are genocidal".

I'm not the one murdering them, quite literally. Just like in real life, there is no mystical unstoppable force of nature in play. It's another person, like you. Their choices aren't your choices.

To put it another way, if you sold a kid a bike and he later crashes and dies despite the bike having no faults, are you responsible? Most would correctly identify that you are not responsible in that scenario, as the kid is responsible for what they did with the bike.