Part of Holmes is aware of how cruel he's being. But Holmes is often cruel for the sake of experiment, and he's certain Sherlock is, too.

He isn't injured, so he's up fast, and he opts to punch Sherlock across the cheek--not quite hard enough to fracture any bones, but enough he hopes to stun him out of the panic. It's hard to say for sure how far gone Sherlock's rational mind is.