What is the difference of kill and fillet as compared to ice down and clean it?

They still seem to be holding on, since it has warmed back up here. I hope to haul the fish that are going in the pond on Thursday, and try to fill the tank back up with some fresh water and let the Tilapia clean out and not feed them for a few days then start cleaning a few. I would like to eat a few of them fresh if I can before I freeze the rest of them.