Saul’s plan was to surround the house at night and to have David killed as soon as he came out unsuspectingly in the morning.

And Saul sent messengers to David's house
Supposing that he was gone there; where this was is not said, very likely …

Michal's stratagem to gain time until David got to a distance was allowable, but her falsehood did not even have the plea of necessity to excuse it…