Charles Ellicott Commentary Psalms 139:19

"Surely thou wilt slay the wicked, O God: Depart from me therefore, ye bloodthirsty men." — Psalms 139:19 (ASV)

Slay the wicked.— This abrupt transition from a theme so profound and fascinating to fierce indignation against the enemies of God would certainly be strange anywhere but in the Psalms. And yet, perhaps, philosophically regarded, the subject of God’s omniscience must conduct the mind to the thought of the existence of evil, and speculation on its origin and development. But the Hebrew never speculated for speculation’s sake. The practical concerns of life engaged him too intensely. Where a modern would have branched off into the ever-recurring problem of the entrance of evil into the world, the Israelite turned with indignation on those who then and there proved the existence of sin in concrete act.

Surely ... —Or, rather—

O that you would slay, O God, the wicked,
And that you bloody men would depart from me.

We get the last clause, which is better than an abrupt change to the imprecations, by a slight change of reading.