BloodLust triggers when the enemy is 1 tile away from you, AKA you must be close.
Hey. I tested it with the bow while being on the back of an NPC at the tavern. Bloodlust did not trigger. Over 140 damage, so for certain it was above half the NPC's max HP. Could you test it as well?
Update: Also tested with the Enhanced Dagger's Backstab skill. 90 damage or so to tramps at night. The killing shot did not trigger Bloodlust.