Bloodlust is earned by killing 100 enemies in the game. There are not 100 enemies to kill in a playthrough if you don't grind things out or die enough times to have to replay some enemies multiple times. I have also read reports that you can lose progress to your 100 kills if you choose to start a new game from the menu, so getting all 100 in your first playthrough or following the instructions below to not lose progress will be important.
If you complete the game without getting 100 kills and want to continue another playthrough without resetting kills, make sure to defeat the final boss, walk to the end and watch the credits, and start the story again without using the main menu. I personally needed the 2nd playthrough for my achievement and only needed to kill a few enemies at the very beginning of the story.
If you would like to grind kills for this achievement, the easiest way to do so is to find two enemies together, kill the first one and then die. You will be reset to just before the battle... so you can kill the first enemy, die, kill the first enemy, die, etc. You can repeat this until the achievement pops. You don't need to go crazy an grind kills forever, getting 10-20 extra during your first playthrough should be enough to ensure you get 100 on your first go.
The first time you get to a place to fight 2 enemies at once is about the 15:30 mark in the video below, just after you collect the Light Stone:
UPDATE: "MasterCheeks117" found a great spot to farm 3 EASY kills at a time and get yourself killed to respawn moments away. About 3/4 through the game (46:10 in the Trophy Tom video), you come across two enemies fighting. If you can kill the hostile enemy (you may or may not also need to kill the two passive enemies to the left to get the checkpoint), you can continue left and there is a large hut with an elder/healer and 3 wounded infirm. The checkpoint is just outside the hut. You can walk in and execute a guy on the right side, roll past the now hostile elder, and execute 2 more wounded on the left side, then let the elder kill you. Rinse and repeat.
For all achievements in the game, you can follow that awesome guide on YouTube:
All credit to Trophy Tom. Please like or subscribe to support him. Please note how he has conveniently labeled sections of his video based on which achievement is next or what item you are tracking down, this can be seen if you watch his video on YouTube on the video progress bar instead of the imbedded video here on True Achievements. It is a very well done 100% guide.
*If this solution helped you in some way, please vote positively. If you think it needs improvement, please let me know how in a private message or by leaving a comment.*)