I'm much better at killing them in real life - exceptionally high accuracy with bare hands.
Move in slowly, with hands positioned to clap them above the fly. It helps if you watch for when the fly is cleaning itself, that indicates that it's reasonably at ease, and is less prone to notice your slowly-approaching hands.
When you get a few inches away, clap.
The fly sees your hands approaching and takes off - right into the kill zone.