When wanted bank robbers and their hostages, are taken captive and chained together in a lonely cop’s basement, they learn he’s secretly a serial killer, and must work together to escape before becoming his next victim.

Sorry, but I have to agree with dpg on this one. I think there's something interesting in the "cop who's a serial killer" idea but this story is in desperate need of a single protagonist. You mention something character driven but you have the group working collectively as the hero - considering this group contains criminals and their hostages I'd imagine some conflict here that would require one person to unite them - I want to know what his/her story is.

Ultimately though, you need to figure out how to make it believable first. It could be that it totally works but it's just not coming through in the logline. I'm not sure if it's the story that is unbelievable or simply the logline doesn't quite have the details needed.

How does this cop get all these people in the basement?

Maybe explore the "cop who's a serial killer" idea in a different way?

A single cop is able to capture,? control and corral several robbers and their hostages into his basement???

I, for one, am unable to suspend disbelief.

A single cop is able to capture,? control and corral several robbers and their hostages into his basement???

I, for one, am unable to suspend disbelief.