If anything requiring killing was wrong livelihood, than why wasn't farming banned too?
Towards harvest-time in Magadha, when the crops stand thick in the fields, it is dangerous for the deer in the forests round. Anxious to kill the creatures that devour their crops, the peasants dig pitfalls, fix stakes, set stone-traps, and plant snares and other gins; so that many deer are slain.
From Access to Insight
One day, however, when Pipphali Kassapa was inspecting the fields, it happened that he saw, as if with new eyes, what he had seen so often before. He observed that when his people plowed, many birds gathered and eagerly picked the worms from the furrows. This sight, so common to a farmer, now startled him. It now struck him forcefully that what brought him his wealth, the produce of his fields, was bound up with the suffering of other living beings. His livelihood was purchased with the death of so many worms and other little creatures living in the soil. Thinking about this, he asked one of his laborers: "Who will have to bear the consequences of such an action?" — "You yourself, sir," was the answer.4