When he was down the way he was, it wasn't difficult to avoid Sirius' last attack. Severus felt no real pleasure in the ease of the gesture it took to brush that spell away. He knew for a fact that Black had once been a brilliant duelist, quick on his feet and with his wand.
This was probably a lack of practice now, Snape getting the immediate upperhand in too close of quarters. He wasn't unskilled himself, but this had been... too easy, perhaps.
But he finished it all the same.
And once it was all said and done, and he was surrounded by the mess of the room, overturned furniture and blood -- Snape couldn't say he felt happy about that either.
He'd thought there'd be more than this. Something better. Maybe that would take time to sink in.