“and Yahweh sent you on a mission.” The Hebrew word translated “mission” is derek (#01870 דֶּרֶךְ), the common word for “road” or “way.” The Bible has a lot to say about the “road” that we are to travel on; the straight road, without turning to the right or to the left. Yahweh sent Saul on a “road” for him to follow, in this case a specific “mission,” but Saul turned off the road and went his own way. In this case, the word “road” is translated “mission” due to its being a specific “road” to take (cp. CJB; CSB; ESV; NAB; NASB; NIV; NLT). Some versions translate the Hebrew as “journey,” but that does not catch the meaning as well as “mission.”
“devoted to destruction.” That is, kill them all. [For more on things “devoted” to Yahweh and devoted to destruction, see commentary on Josh. 6:17].