You can unlock this achievement easily (and obviously) by playing through the Seasaon 2 campaign.
Not so obviously, you can also unlock this achievement without ever playing Season 2. Simply play Tarot missions, which you need to do for another achievement anyway, and many times the mission will be populated with Khorne enemies. With 5 or 6 such Tarot missions, you should get enough Khorne kills to unlock this.
Occasionally you will also face Khorne enemies when playing random Priority Assignments, and in some of the DLC Priority Assignments that become playable once you have unlock the Agartha subsector, which I believe requires a 1300 power level.