“for like an angel of God, so is my lord the king.” The woman knew that angels were messengers who represented God, and she sees David “like an angel” here, in the sense that what he says and does are representing God.
“good and bad.” The woman was almost certainly using “good and bad” to indicate the whole spectrum of life, “good and bad and everything in between.” Technically, that is the figure of speech polarmerismos.
[For more on polarmerismos, see commentary on Joshua 14:11.]