I don't see Melanie Lynskey as a warlord. Maybe she'll prove me wrong. It does seem odd she's choosing to ignore the heaving, undulating floor. The person, whom I have no clue who he is, is really that important? And I'll hazard a guess that the boy at the end is Sam.
She's obviously consumed by anger. She instantly interprets Joels arrival as the actions of the bloke she's hunting, kills a Doctor (in the zombie apocalypse no less) out of spite, wastes manpower conducting a room to room search of a whole city looking for the bloke she's after, and then when shown the room the bloke she was after was hiding in, ignores the Zombie issue because she feels like she's super close to finding him.