On his quick exit from the scene, and (as far as we know at this point) not going into other rooms and survival of the woman I think we need to parse what we know in hindsight from what he would have been presented with at the time. So the question of why he did not go on to continue killing includes following considerations:
So he has gone into two rooms and found, quite possibly by total surprise, that instead of a single female in each, there are two persons on each room. We can't know what he thought or knew, but we do know there are four more bedrooms. If he knew there aware three or four more bedrooms than the two he went into, he could be concerned that there maybe up to eight additional persons remaining alive in that house. Moreover, due to noise that already occurred, and more noise that was likely to occur if he kept killing -- that surviving people would also be in more roused state than the people he killed had been. That he could be forced into forcing doors against resistance, and perhaps being faced with one, two, three, or four alerted and standing military age males, or perhaps even armed people. Finally that people would be calling cops.
So danger to his person, as well as risk of leaving evidence, would be increased as he faced meaningful resistance.
At the point he fled, for all he knew, police had already been called, and more people, up to eight given the average number he had faced, were awake and likely his ambush advantage would have been gone.