In the traditional Hebrew text, this verse has the form of a question. The Dead Sea Scroll of Habakkuk treats it as a statement by omitting one letter. This matches the Septuagint but is not followed by any major modern translation. A question certainly seems more appropriate in the context: Habakkuk asks how long the LORD will allow Babylonian cruelty to continue.
Emptying his net continues the picture of the Babylonian armies as fishermen from verses 14-16. Here they are described as taking the fish they have caught out of their net. The Dead Sea scroll is again one letter different from the traditional text and has “sword” instead of “net.” This is followed by Good News Translation with “Are they going to use their swords” (compare Hebrew Old Testament Text Project, New American Bible, New English Bible, Bible en français courant, Die Bibel im heutigen Deutsch). There is little difference in overall meaning between the two possibilities. The Dead Sea Scroll text gives in nonfigurative language the same sense that the traditional text gives in a figure of speech. The majority of modern versions (Revised Standard Version, Bible de Jérusalem, Jerusalem Bible, Traduction œcuménique de la Bible, New International Version, New Jerusalem Bible) follow the traditional text. This seems the best thing to do, since it is in keeping with the metaphor of the previous three verses.
The word translated for ever goes with the second half of the verse in the traditional Hebrew text as in Revised Standard Version, but it goes with the first half in the Dead Sea Scroll, as in Good News Translation. In a sense for ever goes with both halves of the verse anyway. Translators may place it in either half, according to the stylistic requirements of their language.
Slaying nations is expressed in more modern language by Good News Translation as “destroying nations.” Mercilessly means “without showing any pity.”
Quoted with permission from Clark, David J. & Hatton, Howard A. A Handbook on the Book of Habakkuk. (UBS Helps for Translators). New York: UBS, 1989. For this and other handbooks for translators see here .