Why were the men killed?

In my case, I thought it echoed a Soylent Green theme, with that conveyor/sluice way.

It's not clear what they'd use the smushed innards for, but some insects and plants absorb individual victims that way. So, there's that parallel.

In this case, I thought there was a kind of mass production. In other words, other "ScarJos" in other towns, and their victims' innards joining into that sluice way. If they were extra-dimensional, or just super-advanced, they could make that happen.

I also thought it was a metaphor for how politicians seduce us and then suck out the innards of our wallets... if not more!

For me, I didn't have to figure out every detail. The overall effect and situations drew me in.

They were killed for their meat and transported to the aliens' homeworld in the book. IMO we get more than enough clues to make this connection in the film. The manner of their deaths and the fact they get sort of fattened-up first, and the image of meat on a conveyor belt sliding into a rift of some kind all suggest lambs for the slaughter.

You do get more information in the book about human meat being a delicacy for the moneyed elite of the homeworld, but I enjoy the film's minimalist take on the main plot.