I'm not sure we can say that they haven't already been executing civilians. Information leaving the country is quite limited, and there have been reports of Taliban going house to house looking for certain people in some areas, Taliban in some places demanding daughters be turned over to them to be their wives and the bbc had Interviewed an internal refugee a couple of days ago who said the Taliban had executed men and boys in her village.
It may look quiet on the streets of Kabul where the news reporters are, and the Taliban spokesmen may be talking about a peaceful transition etc, but I don't think that means that these things aren't going on behind the scenes or in more remote areas.
Even if there were large scale executions going on right outside the airport I don't think our armies would do anything - governments have made it quite clear that we're abandoning the country come what may.
Agree it makes sense for the Taliban to hold off attacking the airport and our forces. As long as they don't do something to provoke a response our government will leave them to it, but if they did attack I think its fair to say the recent halt to air strikes would quickly be reversed.