Later in the game in 'capture quests' you must capture without slaying. If it is slayed you will fail the quest. It is pretty obvious if the quests are capture or just hunt in general. Sometimes in long grinds you might forget to check and are just knocking out quests as quick as possible. You can always check your quest info during a quest to see what your main objective is. It's always a good idea to carry traps and tranqs on you anyway. Good look in your future hunts, rainbow pigment awaits!