He can't tell what she was doing; nothing good, likely enough. But with her not running, something else needs to be done, something else to emphasize how outmatched she is, whether or not she actually is-

And while he has no idea what a 'Heartless' is, there's another step taken. Then another. Then he's broken out in to an outright run, rushing her and looking as though he's going to completely bowl in to and over her, oversized key-sword or not.